SCHEDULE 13G

Amendment No. 2
AMBAC Financial Group Incorporated
Common Stock
Cusip #023139108


Cusip #023139108
Item 1:	Reporting Person - FMR LLC
Item 4:	Delaware
Item 5:	340,600
Item 6:	0
Item 7:	17,976,015
Item 8:	0
Item 9:	17,976,015
Item 11:	6.258%
Item 12:	    HC


Cusip #023139108
Item 1:	Reporting Person - Edward C. Johnson 3d
Item 4:	United States of America
Item 5:	0
Item 6:	0
Item 7:	17,976,015
Item 8:	0
Item 9:	17,976,015
Item 11:	6.258%
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:

		AMBAC Financial Group Incorporated

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

		One State Street Plaza
		New York, NY  10004


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:

		023139108

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:	17,976,015

	(b)	Percent of Class:	6.258%

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

	(i)	sole power to vote or to direct
the vote:	340,600

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

	(iii)	sole power to dispose or to
direct the disposition of:	17,976,015

	(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 AMBAC Financial Group
Incorporated.  The interest of one person, Fidelity Growth &
Income Fund, an investment company registered under the
Investment Company Act of 1940, in the Common Stock of
AMBAC Financial Group Incorporated, amounted to
15,000,000 shares or 5.222% of the total outstanding Common
Stock at November 30, 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.

December 09, 2008
Date

 /s/ Scott C. Goebel
Signature

Scott C. Goebel
Duly authorized under Power of Attorney
effective as of June 1, 2008 by and on behalf of 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 17,088,715 shares or
5.949% of the Common Stock outstanding of AMBAC
Financial Group 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 & Income Fund, amounted to 15,000,000 shares or
5.222% of the Common Stock outstanding. Fidelity Growth &
Income 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 17,088,715 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.

	Strategic Advisers, Inc., 82 Devonshire Street, Boston,
MA 02109, a wholly-owned subsidiary of FMR LLC and an
investment adviser registered under Section 203 of the
Investment Advisers Act of 1940, provides investment
advisory services to individuals.  As such, FMR LLC's
beneficial ownership includes 338,400 shares, or 0.118%, of
the Common Stock stock outstanding of AMBAC Financial
Group Incorporated, beneficially owned through Strategic
Advisers, Inc.

	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 548,900 shares or 0.191% of the
outstanding Common Stock of the AMBAC Financial Group
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 548,900 shares and sole power to
vote or to direct the voting of 340,600 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 December 09, 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 AMBAC Financial Group Incorporated at
November 30, 2008.

	FMR LLC

	By /s/ Scott C. Goebel
	Scott C. Goebel
	Duly authorized under Power of Attorney effective as
of June 1, 2008, by and on behalf of FMR LLC and its direct
and indirect subsidiaries

	Edward C. Johnson 3d

	By /s/ Scott C. Goebel
	Scott C. Goebel
	Duly authorized under Power of Attorney effective as
of June 1, 2008, by and on behalf of Edward C. Johnson 3d

	Fidelity Management & Research Company

	By /s/ Scott C. Goebel
	Scott C. Goebel
	Senior V.P. and General Counsel

	Fidelity Growth & Income Fund

	By /s/ Scott C. Goebel
	Scott C. Goebel
	Secretary