SCHEDULE 13G

Amendment No. 4
Synaptics Incorporated
Common Stock
Cusip #87157D109


Cusip #87157D109
Item 1:	Reporting Person - FMR LLC
Item 4:	Delaware
Item 5:	44,615
Item 6:	0
Item 7:	4,624,550
Item 8:	0
Item 9:	4,624,550
Item 11:	19.235%
Item 12:	    HC


Cusip #87157D109
Item 1:	Reporting Person - Edward C. Johnson 3d
Item 4:	United States of America
Item 5:	0
Item 6:	0
Item 7:	4,624,550
Item 8:	0
Item 9:	4,624,550
Item 11:	19.235%
Item 12:	IN




	SCHEDULE 13G - TO BE INCLUDED IN
STATEMENTS
	FILED PURSUANT TO RULE 13d-1(b) or 13d-2(b)



Item 1(a).	Name of Issuer:

		Synaptics Incorporated

Item 1(b).	Name of Issuer's Principal Executive Offices:

		3120 Scott Blvd
		Suite 130
		Santa Clara, CA  95054


Item 2(a).	Name of Person Filing:

		FMR LLC

Item 2(b).	Address or Principal Business Office or, if None,
Residence:

		82 Devonshire Street, Boston,
Massachusetts  02109

Item 2(c).	Citizenship:

		Not applicable

Item 2(d).	Title of Class of Securities:

		Common Stock

Item 2(e).	CUSIP Number:

		87157D109

Item 3.	This statement is filed pursuant to Rule 13d-1(b) or 13d-2(b)
and the person filing, FMR LLC, is a parent holding company
in accordance with Section 240.13d-1(b)(ii)(G).  (Note:  See
Item 7).

Item 4.	Ownership

	(a)	Amount Beneficially Owned:	4,624,550

	(b)	Percent of Class:	19.235%

	(c)	Number of shares as to which such
person has:

	(i)	sole power to vote or to direct
the vote:	44,615

	(ii)	shared power to vote or to
direct the vote:	0

	(iii)	sole power to dispose or to
direct the disposition of:	4,624,550

	(iv)	shared power to dispose or to
direct the disposition of:	0



Item 5.	Ownership of Five Percent or Less of a Class.

	Not applicable.

Item 6.	Ownership of More than Five Percent on Behalf of Another
Person.

	Various persons have the right to receive or the power to
direct the receipt of dividends from, or the proceeds from the
sale of, the Common Stock of Synaptics Incorporated.  The
interest of one person, Fidelity Growth Company Fund, an
investment company registered under the Investment
Company Act of 1940, in the Common Stock of Synaptics
Incorporated, amounted to 3,085,082 shares or 12.832% of the
total outstanding Common Stock at February 29, 2008.

Item 7.	Identification and Classification of the Subsidiary Which
Acquired the Security Being Reported on By the Parent
Holding Company.

	See attached Exhibit A.

Item 8.	Identification and Classification of Members of
the Group.

	Not applicable.  See attached Exhibit A.

Item 9.	Notice of Dissolution of Group.

	Not applicable.

Item 10.	Certification.

	By signing below I certify that, to the best of my knowledge
and belief, the securities referred to above were acquired in the
ordinary course of business and were not acquired for the
purpose of and do not have the effect of changing or
influencing the control of the issuer of such securities and
were not acquired in connection with or as a participant in any
transaction having such purpose or effect.

Signature

	After reasonable inquiry and to the best of my knowledge and
belief, I certify that the information set forth in this statement
is true, complete and correct.

March 07, 2008
Date

/s/Joseph Mari
Signature

Joseph Mari
Duly authorized under Power of Attorney
dated March 5, 2008 by and on behalf of FMR Corp. (now
known as FMR LLC) and its direct and indirect subsidiaries


	SCHEDULE 13G - TO BE INCLUDED IN
STATEMENTS
	FILED PURSUANT TO RULE 13d-1(b) or 13d-2(b)



	Pursuant to the instructions in Item 7 of Schedule 13G,
Fidelity Management & Research Company ("Fidelity"), 82
Devonshire Street, Boston, Massachusetts 02109, a wholly-
owned subsidiary of FMR LLC and an investment adviser
registered under Section 203 of the Investment Advisers Act
of 1940, is the beneficial owner of 4,572,935 shares or
19.020% of the Common Stock outstanding of Synaptics
Incorporated  ("the Company") as a result of acting as
investment adviser to various investment companies registered
under Section 8 of the Investment Company Act of 1940.

	The ownership of one investment company, Fidelity
Growth Company Fund, amounted to 3,085,082 shares or
12.832% of the Common Stock outstanding. Fidelity Growth
Company Fund has its principal business office at 82
Devonshire Street, Boston, Massachusetts 02109.

	Edward C. Johnson 3d and FMR LLC, through its
control of Fidelity, and the funds each has sole power to
dispose of the 4,572,935 shares owned by the Funds.

	Members of the family of Edward C. Johnson 3d,
Chairman of FMR LLC, are the predominant owners, directly
or through trusts, of Series B voting common shares of FMR
LLC, representing 49% of the voting power of FMR LLC.
The Johnson family group and all other Series B shareholders
have entered into a shareholders' voting agreement under
which all Series B voting common shares will be voted in
accordance with the majority vote of Series B voting common
shares.  Accordingly, through their ownership of voting
common shares and the execution of the shareholders' voting
agreement, members of the Johnson family may be deemed,
under the Investment Company Act of 1940, to form a
controlling group with respect to FMR LLC.

	Neither FMR LLC nor Edward C. Johnson 3d,
Chairman of FMR LLC, has the sole power to vote or direct
the voting of the shares owned directly by the Fidelity Funds,
which power resides with the Funds' Boards of Trustees.
Fidelity carries out the voting of the shares under written
guidelines established by the Funds' Boards of Trustees.

	Pyramis Global Advisors, LLC ("PGALLC"), 53 State
Street, Boston, Massachusetts, 02109, an indirect wholly-
owned subsidiary of FMR LLC and an investment adviser
registered under Section 203 of the Investment Advisers Act
of 1940, is the beneficial owner of 6,900 shares or 0.029% of
the outstanding Common Stock of Synaptics Incorporated as a
result of its serving as investment adviser to institutional
accounts, non-U.S. mutual funds, or investment companies
registered under Section 8 of the Investment Company Act of
1940 owning such shares.

	Edward C. Johnson 3d and FMR LLC, through its
control of PGALLC, each has sole dispositive power over
6,900 shares and sole power to vote or to direct the voting of
6,900 shares of Common Stock owned by the institutional
accounts or funds advised by PGALLC as reported above.

	Pyramis Global Advisors Trust Company ("PGATC"),
53 State Street, Boston, Massachusetts, 02109, an indirect
wholly-owned subsidiary of FMR LLC and a bank as defined
in Section 3(a)(6) of the Securities Exchange Act of 1934, is
the beneficial owner of 12,600 shares or 0.052% of the
outstanding Common Stock of the Synaptics Incorporated as a
result of its serving as investment manager of institutional
accounts owning such shares.

	Edward C. Johnson 3d and FMR LLC, through its
control of Pyramis Global Advisors Trust Company, each has
sole dispositive power over 12,600 shares and sole power to
vote or to direct the voting of 0 shares of Common Stock
owned by the institutional accounts managed by PGATC as
reported above.

	FIL Limited, formerly known as Fidelity International
Limited ("FIL"), Pembroke Hall, 42 Crow Lane, Hamilton,
Bermuda, and various foreign-based subsidiaries provide
investment advisory and management services to a number of
non-U.S. investment companies and certain institutional
investors.  FIL, which is a qualified institution under section
240.13d-1(b)(1) pursuant to an SEC No-Action letter dated
October 5, 2000, is the beneficial owner of 32,115 shares or
0.134% of the Common Stock outstanding of the Company.

	Partnerships controlled predominantly by members of
the family of Edward C. Johnson 3d, Chairman of FMR LLC
and FIL, or trusts for their benefit, own shares of FIL voting
stock with the right to cast approximately 47% of the total
votes which may be cast by all holders of FIL voting stock.
FMR LLC and FIL are separate and independent corporate
entities, and their Boards of Directors are generally composed
of different individuals.

	FMR LLC and FIL are of the view that they are not
acting as a "group" for purposes of Section 13(d) under the
Securities Exchange Act of 1934 (the "1934" Act) and that
they are not otherwise required to attribute to each other the
"beneficial ownership" of securities "beneficially owned" by
the other corporation within the meaning of Rule 13d-3
promulgated under the 1934 Act.  Therefore, they are of the
view that the shares held by the other corporation need not be
aggregated for purposes of Section 13(d). However, FMR
LLC is making this filing on a voluntary basis as if all of the
shares are beneficially owned by FMR LLC and FIL on a joint
basis.



	SCHEDULE 13G - TO BE INCLUDED IN
STATEMENTS
	FILED PURSUANT TO RULE 13d-1(b) or 13d-2(b)
	RULE 13d-1(f)(1)  AGREEMENT

	The undersigned persons, on March 07, 2008, agree
and consent to the joint filing on their behalf of this Schedule
13G in connection with their beneficial ownership of the
Common Stock of Synaptics Incorporated at February 29,
2008.

	FMR LLC

	By /s/ Joseph Mari
	Joseph Mari
	Duly authorized under Power of Attorney dated March
5, 2008, by Eric D. Roiter by and on behalf of FMR Corp.
(now known as FMR LLC) and its direct and indirect
subsidiaries

	Edward C. Johnson 3d

	By /s/ Joseph Mari
	Joseph Mari
	Duly authorized under Power of Attorney dated March
5, 2008, by Eric D. Roiter by and on behalf of Edward C.
Johnson 3d

	Fidelity Management & Research Company

	By /s/ Joseph Mari
	Joseph Mari
	Duly authorized under Power of Attorney dated March
5, 2008, by Eric D. Roiter Senior V.P. and General Counsel

	Fidelity Growth Company Fund

	By /s/ Joseph Mari
	Joseph Mari
	Duly authorized under Power of Attorney dated March
5, 2008, by Eric D. Roiter Secretary











SPECIAL NOTE REGARDING FORWARD-LOOKING STATEMENTS

This website contains forward-looking statements that are subject to the safe harbors created under the Securities Act of 1933, as amended, and the Securities Exchange Act of 1934, as amended. Forward-looking statements give our current expectations and projections relating to our financial condition, results of operations, plans, objectives, future performance and business, and can be identified by the fact that they do not relate strictly to historical or current facts. Such forward-looking statements may include words such as "expect," "anticipate," "intend," "believe," "estimate," "plan," "target," "strategy," "continue," "may," "will," "should," variations of such words, or other words and terms of similar meaning. All forward-looking statements reflect our best judgment and are based on several factors relating to our operations and business environment, all of which are difficult to predict and many of which are beyond our control. Such factors include, but are not limited to, the risks as identified in the "Risk Factors," "Management's Discussion and Analysis of Financial Condition and Results of Operations" and "Business" sections of our Annual Report on Form 10-K for our most recent fiscal year, and other risks as identified from time to time in our Securities and Exchange Commission reports. Forward-looking statements are based on information available to us on the date hereof, and we do not have, and expressly disclaim, any obligation to publicly release any updates or any changes in our expectations, or any change in events, conditions, or circumstances on which any forward-looking statement is based. Our actual results and the timing of certain events could differ materially from the forward-looking statements. These forward-looking statements do not reflect the potential impact of any mergers, acquisitions, or other business combinations that had not been completed as of the date of this filing.