Schedule II - Condensed Financial Information of Registrant (Parent Company Only) - Condensed Balance Sheet (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Assets:
Short-term investments, at cost (approximates fair value)	$ 661,219	$ 783,015
Cash	43,837	15,999
Restricted cash		2,500
Other assets	39,715	104,300
Total assets	27,007,164	27,113,695
Liabilities:
Liabilities subject to compromise	1,704,904	1,707,421
Current taxes	96,778	95,709
Total liabilities	30,254,131	30,263,228
Stockholders' deficit:
Preferred stock, par value $0.01 per share; authorized shares-4,000,000; issued and outstanding shares-none
Common stock, par value $0.01 per share; authorized shares-650,000,000 at December 31, 2012 and 2011; issued and outstanding shares-308,016,764 at December 31, 2012 and 2011	3,080	3,080
Additional paid-in capital	2,172,027	2,172,027
Accumulated other comprehensive income	625,385	463,259
Accumulated deficit	(6,297,264)	(6,039,922)
Common stock held in treasury at cost, 5,580,657 shares at December 31, 2012 and 5,587,953 shares at December 31, 2011	(410,755)	(411,419)
Total Ambac Financial Group, Inc. stockholders' deficit	(3,907,527)	(3,812,975)
Total liabilities and stockholders' deficit	27,007,164	27,113,695
AMBAC FINANCIAL GROUP, INC PARENT COMPANY ONLY [Member]
Assets:
Short-term investments, at cost (approximates fair value)	30,316	34,899
Cash	41	496
Restricted cash		2,500
Other assets	6,452	6,481
Total assets	(2,183,915)	(2,087,743)
Liabilities:
Liabilities subject to compromise	1,704,939	1,707,456
Current taxes	1,900	1,900
Accounts payable and other liabilities	16,773	15,876
Total liabilities	1,723,612	1,725,232
Stockholders' deficit:
Preferred stock, par value $0.01 per share; authorized shares-4,000,000; issued and outstanding shares-none
Common stock, par value $0.01 per share; authorized shares-650,000,000 at December 31, 2012 and 2011; issued and outstanding shares-308,016,764 at December 31, 2012 and 2011	3,080	3,080
Additional paid-in capital	2,172,027	2,172,027
Accumulated other comprehensive income	625,385	463,259
Accumulated deficit	(6,297,264)	(6,039,922)
Common stock held in treasury at cost, 5,580,657 shares at December 31, 2012 and 5,587,953 shares at December 31, 2011	(410,755)	(411,419)
Total Ambac Financial Group, Inc. stockholders' deficit	(3,907,527)	(3,812,975)	(2,008,536)
Total liabilities and stockholders' deficit	(2,183,915)	(2,087,743)
AMBAC FINANCIAL GROUP, INC PARENT COMPANY ONLY [Member] | Non-debtors subsidiaries [Member]
Assets:
Investments in non-debtor subsidiaries	$ (2,220,724)	$ (2,132,119)