SCHEDULE 13G

Amendment No. 2
Illumina Inc.
Common Stock
Cusip #452327109


Cusip #452327109
Item 1:	Reporting Person - FMR LLC
Item 4:	Delaware
Item 5:	900
Item 6:	0
Item 7:	8,342,595
Item 8:	0
Item 9:	8,342,595
Item 11:	14.990%
Item 12:	    HC


Cusip #452327109
Item 1:	Reporting Person - Edward C. Johnson 3d
Item 4:	United States of America
Item 5:	0
Item 6:	0
Item 7:	8,342,595
Item 8:	0
Item 9:	8,342,595
Item 11:	14.990%
Item 12:	IN




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

This Amendment to Schedule 13G filed by FMR LLC amends
the statement on Schedule 13G previously filed by FMR
Corp., the predecessor of FMR LLC.


Item 1(a).	Name of Issuer:

		Illumina Inc.

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

		9885 Towne Centre Drive
		San Diego, CA  92121


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:

		452327109

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:	8,342,595

	(b)	Percent of Class:	14.990%

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

	(i)	sole power to vote or to direct
the vote:	900

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

	(iii)	sole power to dispose or to
direct the disposition of:	8,342,595

	(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 Illumina Inc..  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 Illumina Inc., amounted to
3,315,839 shares or 5.958% of the total outstanding Common
Stock at December 31, 2007.

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 13, 2008
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.
(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 8,341,695 shares or
14.988% of the Common Stock outstanding of Illumina Inc.
("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 number of shares
of Common Stock of Illumina Inc. owned by the investment
companies at December 31, 2007 included 155,803 shares of
Common Stock resulting from the assumed conversion of
452,917 shares of ILLUMINA INC WT 1/19/11 (0.344 shares
of Common Stock for each share of Convertible Preferred
Stock). The number of shares of Common Stock of Illumina
Inc. owned by the investment companies at December 31,
2007 included 122,043 shares of Common Stock resulting
from the assumed conversion of 354,776 shares of
ILLUMINA INC WT 11/22/10 (0.344 shares of Common
Stock for each share of Convertible Preferred Stock).

	The ownership of one investment company, Fidelity
Growth Company Fund, amounted to 3,315,839 shares or
5.958% 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 8,341,695 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 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 900 shares or 0.002% of the
outstanding Common Stock of the Illumina Inc. 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 900 shares and sole power to vote
or to direct the voting of 900 shares of Common Stock owned
by the institutional accounts managed by PGATC 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 13, 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 Illumina Inc. at December 31, 2007.

	FMR LLC

	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. (now
known as FMR LLC) 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

	Fidelity Growth Company Fund

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