Outstanding Exposure (Details 2) (USD $)	3 Months Ended
	Mar. 31, 2012 Transaction	Dec. 31, 2011
Outstanding Exposure
Net mortgage guaranty insurance in force	$ 176,700,000
Net Par Outstanding	567,098,000,000	558,048,000,000
% of Total Net Par Outstanding	100.00%	100.00%
Super senior
Outstanding Exposure
Net Par Outstanding	22,140,000,000	23,554,000,000
% of Total Net Par Outstanding	3.90%	4.20%
AAA
Outstanding Exposure
Net Par Outstanding	51,575,000,000	52,422,000,000
% of Total Net Par Outstanding	9.10%	9.40%
AA
Outstanding Exposure
Net Par Outstanding	157,433,000,000	154,300,000,000
% of Total Net Par Outstanding	27.70%	27.70%
AA | Bank of America agreement
Outstanding Exposure
Percentage of the first lien transactions covered by the Bank of America Agreement reclassified to AA	80.00%
Number of transactions insured	21
Decrease of net outstanding par rated of the first lien transactions covered by the Bank of America Agreement reclassified to AA		1,452,000,000
Net outstanding par rated change if reclassification not done	1,382,000,000
A
Outstanding Exposure
Net Par Outstanding	236,369,000,000	230,541,000,000
% of Total Net Par Outstanding	41.70%	41.30%
BBB
Outstanding Exposure
Net Par Outstanding	73,582,000,000	70,469,000,000
% of Total Net Par Outstanding	13.00%	12.60%
Below investment grade ("BIG")
Outstanding Exposure
Net Par Outstanding	25,999,000,000	26,762,000,000
% of Total Net Par Outstanding	4.60%	4.80%
Public finance
Outstanding Exposure
Net Par Outstanding	456,412,000,000	442,119,000,000
Outstanding commitments to provide guaranties	1,000,000,000
Outstanding commitments to provide guaranties, amount expiring prior to December 31, 2012	700,000,000
Public finance | Minimum
Outstanding Exposure
Expiration date for insured financial obligation commitments	Apr. 01, 2012
Public finance | Maximum
Outstanding Exposure
Expiration date for insured financial obligation commitments	Feb. 25, 2017
Public finance U.S.
Outstanding Exposure
Net Par Outstanding	416,499,000,000	403,073,000,000
% of Total Net Par Outstanding	100.00%	100.00%
Public finance U.S. | AAA
Outstanding Exposure
Net Par Outstanding	4,931,000,000	5,074,000,000
% of Total Net Par Outstanding	1.20%	1.30%
Public finance U.S. | AA
Outstanding Exposure
Net Par Outstanding	144,987,000,000	139,693,000,000
% of Total Net Par Outstanding	34.80%	34.60%
Public finance U.S. | A
Outstanding Exposure
Net Par Outstanding	219,095,000,000	213,164,000,000
% of Total Net Par Outstanding	52.60%	52.90%
Public finance U.S. | BBB
Outstanding Exposure
Net Par Outstanding	42,916,000,000	40,635,000,000
% of Total Net Par Outstanding	10.30%	10.10%
Public finance U.S. | Below investment grade ("BIG")
Outstanding Exposure
Net Par Outstanding	4,570,000,000	4,507,000,000
% of Total Net Par Outstanding	1.10%	1.10%
Public finance Non-U.S.
Outstanding Exposure
Net Par Outstanding	39,913,000,000	39,046,000,000
% of Total Net Par Outstanding	100.00%	100.00%
Public finance Non-U.S. | Super senior
Outstanding Exposure
Net Par Outstanding	1,165,000,000	1,138,000,000
% of Total Net Par Outstanding	2.90%	2.90%
Public finance Non-U.S. | AAA
Outstanding Exposure
Net Par Outstanding	1,384,000,000	1,381,000,000
% of Total Net Par Outstanding	3.50%	3.50%
Public finance Non-U.S. | AA
Outstanding Exposure
Net Par Outstanding	973,000,000	1,056,000,000
% of Total Net Par Outstanding	2.40%	2.70%
Public finance Non-U.S. | A
Outstanding Exposure
Net Par Outstanding	11,126,000,000	11,744,000,000
% of Total Net Par Outstanding	27.90%	30.10%
Public finance Non-U.S. | BBB
Outstanding Exposure
Net Par Outstanding	22,913,000,000	21,399,000,000
% of Total Net Par Outstanding	57.40%	54.80%
Public finance Non-U.S. | Below investment grade ("BIG")
Outstanding Exposure
Net Par Outstanding	2,352,000,000	2,328,000,000
% of Total Net Par Outstanding	5.90%	6.00%
Structured Finance
Outstanding Exposure
Outstanding commitments to provide guaranties	2,100,000,000
Structured finance U.S.
Outstanding Exposure
Net Par Outstanding	87,784,000,000	92,234,000,000
% of Total Net Par Outstanding	100.00%	100.00%
Structured finance U.S. | Super senior
Outstanding Exposure
Net Par Outstanding	15,756,000,000	16,756,000,000
% of Total Net Par Outstanding	18.00%	18.20%
Structured finance U.S. | AAA
Outstanding Exposure
Net Par Outstanding	34,974,000,000	35,736,000,000
% of Total Net Par Outstanding	39.80%	38.70%
Structured finance U.S. | AA
Outstanding Exposure
Net Par Outstanding	10,537,000,000	12,575,000,000
% of Total Net Par Outstanding	12.00%	13.60%
Structured finance U.S. | A
Outstanding Exposure
Net Par Outstanding	4,759,000,000	4,115,000,000
% of Total Net Par Outstanding	5.40%	4.50%
Structured finance U.S. | BBB
Outstanding Exposure
Net Par Outstanding	4,726,000,000	5,044,000,000
% of Total Net Par Outstanding	5.40%	5.50%
Structured finance U.S. | Below investment grade ("BIG")
Outstanding Exposure
Net Par Outstanding	17,032,000,000	18,008,000,000
% of Total Net Par Outstanding	19.40%	19.50%
Structured finance Non-U.S.
Outstanding Exposure
Net Par Outstanding	22,902,000,000	23,695,000,000
% of Total Net Par Outstanding	100.00%	100.00%
Structured finance Non-U.S. | Super senior
Outstanding Exposure
Net Par Outstanding	5,219,000,000	5,660,000,000
% of Total Net Par Outstanding	22.80%	23.90%
Structured finance Non-U.S. | AAA
Outstanding Exposure
Net Par Outstanding	10,286,000,000	10,231,000,000
% of Total Net Par Outstanding	44.90%	43.20%
Structured finance Non-U.S. | AA
Outstanding Exposure
Net Par Outstanding	936,000,000	976,000,000
% of Total Net Par Outstanding	4.10%	4.10%
Structured finance Non-U.S. | A
Outstanding Exposure
Net Par Outstanding	1,389,000,000	1,518,000,000
% of Total Net Par Outstanding	6.10%	6.40%
Structured finance Non-U.S. | BBB
Outstanding Exposure
Net Par Outstanding	3,027,000,000	3,391,000,000
% of Total Net Par Outstanding	13.20%	14.30%
Structured finance Non-U.S. | Below investment grade ("BIG")
Outstanding Exposure
Net Par Outstanding	2,045,000,000	1,919,000,000
% of Total Net Par Outstanding	8.90%	8.10%
Public finance Ireland
Outstanding Exposure
Net mortgage guaranty insurance in force	140,100,000
Public finance UK
Outstanding Exposure
Net mortgage guaranty insurance in force	$ 36,600,000