SCHEDULE 13G

Amendment No. 1
MKS Instruments Incorporated
Common Stock
Cusip #55306N104


Cusip #55306N104
Item 1:	Reporting Person - FMR Corp.
Item 4:	Delaware
Item 5:	792,844
Item 6:	0
Item 7:	6,968,044
Item 8:	0
Item 9:	6,968,044
Item 11:	12.844%
Item 12:	    HC


Cusip #55306N104
Item 1:	Reporting Person - Edward C. Johnson 3d
Item 4:	United States of America
Item 5:	0
Item 6:	0
Item 7:	6,968,044
Item 8:	0
Item 9:	6,968,044
Item 11:	12.844%
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:

		MKS Instruments Incorporated

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

		90 Industrial Way
		Wilmington, MA  01887


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:

		55306N104

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:	6,968,044

	(b)	Percent of Class:	12.844%

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

	(i)	sole power to vote or to direct
the vote:	792,844

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

	(iii)	sole power to dispose or to
direct the disposition of:	6,968,044

	(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 MKS Instruments Incorporated.
The interest of one person, Equity-income Ii, an investment
company registered under the Investment Company Act of
1940, in the Common Stock of MKS Instruments
Incorporated, amounted to 3,588,800 shares or 6.615% of the
total outstanding Common Stock at December 31, 2005.

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, 2006
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 6,565,396 shares or
12.102% of the Common Stock outstanding of MKS
Instruments 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, Equity-
income Ii, amounted to 3,588,800 shares or 6.615% of the
Common Stock outstanding. Equity-income Ii 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 6,565,396 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  402,648 shares or 0.742% 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 402,648 shares and sole power to vote
or to direct the voting of 402,648 shares of Common Stock
owned by the institutional account(s) as reported above.



	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, 2006, 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 MKS Instruments Incorporated at
December 31, 2005.

	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

	Equity-income Ii

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