Outstanding Exposure (Details 2) (USD $)																																																																												12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2013 Bank of America Agreement	Mar. 31, 2013 Super senior	Dec. 31, 2012 Super senior	Mar. 31, 2013 AAA	Dec. 31, 2012 AAA	Mar. 31, 2013 AA	Dec. 31, 2012 AA	Mar. 31, 2013 A	Dec. 31, 2012 A	Mar. 31, 2013 BBB	Dec. 31, 2012 BBB	Mar. 31, 2013 Below investment grade (BIG)	Dec. 31, 2012 Below investment grade (BIG)	Mar. 31, 2013 US Public Finance	Dec. 31, 2012 US Public Finance	Mar. 31, 2013 US Public Finance Super senior	Dec. 31, 2012 US Public Finance Super senior	Mar. 31, 2013 US Public Finance AAA	Dec. 31, 2012 US Public Finance AAA	Mar. 31, 2013 US Public Finance AA	Dec. 31, 2012 US Public Finance AA	Mar. 31, 2013 US Public Finance A	Dec. 31, 2012 US Public Finance A	Mar. 31, 2013 US Public Finance BBB	Dec. 31, 2012 US Public Finance BBB	Mar. 31, 2013 US Public Finance Below investment grade (BIG)	Dec. 31, 2012 US Public Finance Below investment grade (BIG)	Mar. 31, 2013 Public finance Non-U.S.	Dec. 31, 2012 Public finance Non-U.S.	Mar. 31, 2013 Public finance Non-U.S. Super senior	Dec. 31, 2012 Public finance Non-U.S. Super senior	Mar. 31, 2013 Public finance Non-U.S. AAA	Dec. 31, 2012 Public finance Non-U.S. AAA	Mar. 31, 2013 Public finance Non-U.S. AA	Dec. 31, 2012 Public finance Non-U.S. AA	Mar. 31, 2013 Public finance Non-U.S. A	Dec. 31, 2012 Public finance Non-U.S. A	Mar. 31, 2013 Public finance Non-U.S. BBB	Dec. 31, 2012 Public finance Non-U.S. BBB	Mar. 31, 2013 Public finance Non-U.S. Below investment grade (BIG)	Dec. 31, 2012 Public finance Non-U.S. Below investment grade (BIG)	Mar. 31, 2013 US Structured Finance [Member]	Dec. 31, 2012 US Structured Finance [Member]	Mar. 31, 2013 US Structured Finance [Member] Super senior	Dec. 31, 2012 US Structured Finance [Member] Super senior	Mar. 31, 2013 US Structured Finance [Member] AAA	Dec. 31, 2012 US Structured Finance [Member] AAA	Mar. 31, 2013 US Structured Finance [Member] AA	Dec. 31, 2012 US Structured Finance [Member] AA	Mar. 31, 2013 US Structured Finance [Member] A	Dec. 31, 2012 US Structured Finance [Member] A	Mar. 31, 2013 US Structured Finance [Member] BBB	Dec. 31, 2012 US Structured Finance [Member] BBB	Mar. 31, 2013 US Structured Finance [Member] Below investment grade (BIG)	Dec. 31, 2012 US Structured Finance [Member] Below investment grade (BIG)	Mar. 31, 2013 Structured finance Non-U.S.	Dec. 31, 2012 Structured finance Non-U.S.	Mar. 31, 2013 Structured finance Non-U.S. Super senior	Dec. 31, 2012 Structured finance Non-U.S. Super senior	Mar. 31, 2013 Structured finance Non-U.S. AAA	Dec. 31, 2012 Structured finance Non-U.S. AAA	Mar. 31, 2013 Structured finance Non-U.S. AA	Dec. 31, 2012 Structured finance Non-U.S. AA	Mar. 31, 2013 Structured finance Non-U.S. A	Dec. 31, 2012 Structured finance Non-U.S. A	Mar. 31, 2013 Structured finance Non-U.S. BBB	Dec. 31, 2012 Structured finance Non-U.S. BBB	Mar. 31, 2013 Structured finance Non-U.S. Below investment grade (BIG)	Dec. 31, 2012 Structured finance Non-U.S. Below investment grade (BIG)	Mar. 31, 2013 Public finance UK	Mar. 31, 2013 Public finance Ireland	Mar. 31, 2013 Structured finance	Mar. 31, 2013 Public finance	Dec. 31, 2012 Public finance Minimum	Dec. 31, 2012 Public finance Maximum
Financial guarantee obligations
Insured Financial Obligations Outstanding Principal Amount	$ 501,817,000,000	$ 519,893,000,000	$ 3,100,000,000	$ 18,852,000,000	$ 19,576,000,000	$ 38,765,000,000	$ 41,988,000,000	$ 132,201,000,000	$ 135,711,000,000	$ 219,377,000,000	$ 225,984,000,000	$ 70,268,000,000	$ 73,242,000,000	$ 22,354,000,000	$ 23,392,000,000	$ 378,456,000,000	$ 387,967,000,000	$ 0	$ 0	$ 4,053,000,000	$ 4,502,000,000	$ 120,812,000,000	$ 124,525,000,000	$ 206,112,000,000	$ 210,124,000,000	$ 42,890,000,000	$ 44,213,000,000	$ 4,589,000,000	$ 4,603,000,000	$ 35,067,000,000	$ 37,540,000,000	$ 1,071,000,000	$ 1,130,000,000	$ 580,000,000	$ 576,000,000	$ 661,000,000	$ 875,000,000	$ 9,158,000,000	$ 9,781,000,000	$ 21,590,000,000	$ 22,885,000,000	$ 2,007,000,000	$ 2,293,000,000	$ 70,280,000,000	$ 74,695,000,000	$ 13,259,000,000	$ 13,572,000,000	$ 26,628,000,000	$ 28,615,000,000	$ 10,041,000,000	$ 9,589,000,000	$ 3,149,000,000	$ 4,670,000,000	$ 3,351,000,000	$ 3,717,000,000	$ 13,852,000,000	$ 14,532,000,000	$ 18,014,000,000	$ 19,691,000,000	$ 4,522,000,000	$ 4,874,000,000	$ 7,504,000,000	$ 8,295,000,000	$ 687,000,000	$ 722,000,000	$ 958,000,000	$ 1,409,000,000	$ 2,437,000,000	$ 2,427,000,000	$ 1,906,000,000	$ 1,964,000,000
Net mortgage guaranty insurance in force	149,000,000																																																																							14,000,000	135,000,000
% of Total Net Par Outstanding	100.00%	100.00%		3.80%	3.80%	7.70%	8.10%	26.30%	26.10%	43.70%	43.40%	14.00%	14.10%	4.50%	4.50%	100.00%	100.00%	0.00%	0.00%	1.10%	1.20%	31.90%	32.10%	54.50%	54.10%	11.30%	11.40%	1.20%	1.20%	100.00%	100.00%	3.10%	3.00%	1.70%	1.50%	1.90%	2.30%	26.10%	26.10%	61.50%	61.00%	5.70%	6.10%	100.00%	100.00%	18.90%	18.20%	37.90%	38.30%	14.20%	12.80%	4.50%	6.20%	4.80%	5.00%	19.70%	19.50%	100.00%	100.00%	25.10%	24.70%	41.70%	42.10%	3.80%	3.70%	5.30%	7.20%	13.50%	12.30%	10.60%	10.00%
SecuritiesPurchasedForLossMitigationAmountOfGrossPar	1,119,000,000	1,133,000,000
SecuritiesPurchasedToSettleCreditDerivativeContractGrossPar	219,000,000	220,000,000
Outstanding commitments to provide guaranties																																																																										1,800,000,000	700,000,000
Expiration date for insured financial obligation commitments																																																																												Apr. 15, 2013	Feb. 25, 2017
Outstanding commitments to provide guaranties, amount expiring prior to December 31, 2012																																																																											$ 600,000,000