SCHEDULE 13G

Amendment No. 9
Teradyne Incorporated
Common Stock
Cusip #880770102


Cusip #880770102
Item 1:	Reporting Person - FMR Corp.
Item 4:	Delaware
Item 5:	1,470,358
Item 6:	0
Item 7:	28,454,822
Item 8:	0
Item 9:	28,454,822
Item 11:	14.994%
Item 12:	    HC


Cusip #880770102
Item 1:	Reporting Person - Edward C. Johnson 3d
Item 4:	United States of America
Item 5:	0
Item 6:	0
Item 7:	28,454,822
Item 8:	0
Item 9:	28,454,822
Item 11:	14.994%
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:

		Teradyne Incorporated

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

		321 Harrison Avenue

		Boston, MA  02118


Item 2(a).	Name of Person Filing:

		FMR Corp.

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:

		880770102

Item 3.	This statement is filed pursuant to Rule 13d-1(b) or 13d-2(b)
and the person filing, FMR Corp., 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:	28,454,822

	(b)	Percent of Class:	14.994%

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

	(i)	sole power to vote or to direct
the vote:	1,470,358

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

	(iii)	sole power to dispose or to
direct the disposition of:	28,454,822

	(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 Teradyne Incorporated.  The
interest of one person, Magellan Fund, an investment
company registered under the Investment Company Act of
1940, in the Common Stock of Teradyne Incorporated,
amounted to 19,805,264 shares or 10.436% of the total
outstanding Common Stock at December 31, 2006.

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.

February 14, 2007
Date

/s/Eric D. Roiter
Signature

Eric D, Roiter
Duly authorized under Power of Attorney
dated December 30, 1997 by and on behalf of FMR Corp. 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 Corp. and an investment adviser
registered under Section 203 of the Investment Advisers Act
of 1940, is the beneficial owner of 26,990,464 shares or
14.222% of the Common Stock outstanding of Teradyne
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, Magellan
Fund, amounted to 19,805,264 shares or 10.436% of the
Common Stock outstanding. Magellan Fund has its principal
business office at 82 Devonshire Street, Boston,
Massachusetts 02109.

	Edward C. Johnson 3d and FMR Corp., through its
control of Fidelity, and the funds each has sole power to
dispose of the 26,990,464 shares owned by the Funds.

	Members of the family of Edward C. Johnson 3d,
Chairman of FMR Corp., are the predominant owners, directly
or through trusts, of Series B shares of common stock of FMR
Corp., representing 49% of the voting power of FMR Corp.
The Johnson family group and all other Series B shareholders
have entered into a shareholders' voting agreement under
which all Series B shares will be voted in accordance with the
majority vote of Series B shares.  Accordingly, through their
ownership of voting common stock 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 Corp.

	Neither FMR Corp. nor Edward C. Johnson 3d,
Chairman of FMR Corp., 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.

	Fidelity Management Trust Company, 82 Devonshire
Street, Boston, Massachusetts 02109, a wholly-owned
subsidiary of FMR Corp. and a bank as defined in Section
3(a)(6) of the Securities Exchange Act of 1934, is the
beneficial owner of  6,200 shares or 0.003% of the Common
Stock outstanding of the Company as a result of its serving as
investment manager of the institutional account(s).

	Edward C. Johnson 3d and FMR Corp., through its
control of Fidelity Management Trust Company, each has sole
dispositive power over 6,200 shares and sole power to vote or
to direct the voting of 6,200 shares of Common Stock owned
by the institutional account(s) as reported above.

	Strategic Advisers, Inc., 82 Devonshire Street, Boston,
MA 02109, a wholly-owned subsidiary of FMR Corp. and an
investment adviser registered under Section 203 of the
Investment Advisers Act of 1940, provides investment
advisory services to individuals.  As such, FMR Corp.'s
beneficial ownership includes 171,353 shares, or 0.090%, of
the Common Stock stock outstanding of Teradyne
Incorporated, beneficially owned through Strategic Advisers,
Inc.

	Pyramis Global Advisors, LLC ("PGALLC"), 53 State
Street, Boston, Massachusetts, 02109, an indirect wholly-
owned subsidiary of FMR Corp. and an investment adviser
registered under Section 203 of the Investment Advisers Act
of 1940, is the beneficial owner of 328,600 shares or 0.173%
of the outstanding Common Stock of Teradyne 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 Corp., through its
control of PGALLC, each has sole dispositive power over
328,600 shares and sole power to vote or to direct the voting
of 328,600 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 Corp. and a bank as defined
in Section 3(a)(6) of the Securities Exchange Act of 1934, is
the beneficial owner of 405,905 shares or 0.214% of the
outstanding Common Stock of the Teradyne Incorporated as a
result of its serving as investment manager of institutional
accounts owning such shares.

	Edward C. Johnson 3d and FMR Corp., through its
control of Pyramis Global Advisors Trust Company, each has
sole dispositive power over 405,905 shares and sole power to
vote or to direct the voting of 405,905 shares of Common
Stock owned by the institutional accounts managed by
PGATC as reported above.

	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 552,300 shares or 0.291% of the Common Stock
outstanding of the Company.

	Partnerships controlled predominantly by members of
the family of Edward C. Johnson 3d, Chairman of FMR Corp.
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 Corp. and FIL are separate and independent corporate
entities, and their Boards of Directors are generally composed
of different individuals.

	FMR Corp. 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
Corp. is making this filing on a voluntary basis as if all of the
shares are beneficially owned by FMR Corp. 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 February 14, 2007, 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 Teradyne Incorporated at December 31,
2006.

	FMR Corp.

	By /s/ Eric D. Roiter
	Eric D. Roiter
	Duly authorized under Power of Attorney dated
December 30, 1997, by and on behalf of FMR Corp. and its
direct and indirect subsidiaries

	Edward C. Johnson 3d

	By /s/ Eric D. Roiter
	Eric D. Roiter
	Duly authorized under Power of Attorney dated
December 30, 1997, by and on behalf of Edward C. Johnson
3d

	Fidelity Management & Research Company

	By /s/ Eric D. Roiter
	Eric D. Roiter
	Senior V.P. and General Counsel

	Magellan Fund

	By /s/ Eric D. Roiter
	Eric D. Roiter
	Secretary