Outstanding Exposure (Details 2) (USD $)																							12 Months Ended
	Sep. 30, 2013		Dec. 31, 2012		Sep. 30, 2013 US Public Finance		Dec. 31, 2012 US Public Finance		Sep. 30, 2013 Public finance Non-U.S.		Dec. 31, 2012 Public finance Non-U.S.		Sep. 30, 2013 US Structured Finance		Dec. 31, 2012 US Structured Finance		Sep. 30, 2013 Structured finance Non-U.S.		Dec. 31, 2012 Structured finance Non-U.S.		Sep. 30, 2013 Structured finance	Sep. 30, 2013 Public finance	Dec. 31, 2012 Public finance Minimum	Dec. 31, 2012 Public finance Maximum	Sep. 30, 2013 AAA		Dec. 31, 2012 AAA		Sep. 30, 2013 AAA US Public Finance		Dec. 31, 2012 AAA US Public Finance		Sep. 30, 2013 AAA Public finance Non-U.S.		Dec. 31, 2012 AAA Public finance Non-U.S.		Sep. 30, 2013 AAA US Structured Finance		Dec. 31, 2012 AAA US Structured Finance		Sep. 30, 2013 AAA Structured finance Non-U.S.		Dec. 31, 2012 AAA Structured finance Non-U.S.		Sep. 30, 2013 AA		Dec. 31, 2012 AA		Sep. 30, 2013 AA US Public Finance		Dec. 31, 2012 AA US Public Finance		Sep. 30, 2013 AA Public finance Non-U.S.		Dec. 31, 2012 AA Public finance Non-U.S.		Sep. 30, 2013 AA US Structured Finance		Dec. 31, 2012 AA US Structured Finance		Sep. 30, 2013 AA Structured finance Non-U.S.		Dec. 31, 2012 AA Structured finance Non-U.S.		Sep. 30, 2013 A		Dec. 31, 2012 A		Sep. 30, 2013 A US Public Finance		Dec. 31, 2012 A US Public Finance		Sep. 30, 2013 A Public finance Non-U.S.		Dec. 31, 2012 A Public finance Non-U.S.		Sep. 30, 2013 A US Structured Finance		Dec. 31, 2012 A US Structured Finance		Sep. 30, 2013 A Structured finance Non-U.S.		Dec. 31, 2012 A Structured finance Non-U.S.		Sep. 30, 2013 BBB		Dec. 31, 2012 BBB		Sep. 30, 2013 BBB US Public Finance		Dec. 31, 2012 BBB US Public Finance		Sep. 30, 2013 BBB Public finance Non-U.S.		Dec. 31, 2012 BBB Public finance Non-U.S.		Sep. 30, 2013 BBB US Structured Finance		Dec. 31, 2012 BBB US Structured Finance		Sep. 30, 2013 BBB Structured finance Non-U.S.		Dec. 31, 2012 BBB Structured finance Non-U.S.		Sep. 30, 2013 Below investment grade (BIG)		Dec. 31, 2012 Below investment grade (BIG)		Sep. 30, 2013 Below investment grade (BIG) US Public Finance		Dec. 31, 2012 Below investment grade (BIG) US Public Finance		Sep. 30, 2013 Below investment grade (BIG) Public finance Non-U.S.		Dec. 31, 2012 Below investment grade (BIG) Public finance Non-U.S.		Sep. 30, 2013 Below investment grade (BIG) US Structured Finance		Dec. 31, 2012 Below investment grade (BIG) US Structured Finance		Sep. 30, 2013 Below investment grade (BIG) Structured finance Non-U.S.		Dec. 31, 2012 Below investment grade (BIG) Structured finance Non-U.S.		Sep. 30, 2013 securities purchased for loss mitigation purposes net par [Member] Below investment grade (BIG)	Dec. 31, 2012 securities purchased for loss mitigation purposes net par [Member] Below investment grade (BIG)	Sep. 30, 2013 securities purchased for loss mitigation purposes net par [Member] Below investment grade (BIG) US Public Finance	Dec. 31, 2012 securities purchased for loss mitigation purposes net par [Member] Below investment grade (BIG) US Public Finance	Sep. 30, 2013 securities purchased for loss mitigation purposes net par [Member] Below investment grade (BIG) Public finance Non-U.S.	Dec. 31, 2012 securities purchased for loss mitigation purposes net par [Member] Below investment grade (BIG) Public finance Non-U.S.	Sep. 30, 2013 securities purchased for loss mitigation purposes net par [Member] Below investment grade (BIG) US Structured Finance	Dec. 31, 2012 securities purchased for loss mitigation purposes net par [Member] Below investment grade (BIG) US Structured Finance	Sep. 30, 2013 securities purchased for loss mitigation purposes net par [Member] Below investment grade (BIG) Structured finance Non-U.S.	Dec. 31, 2012 securities purchased for loss mitigation purposes net par [Member] Below investment grade (BIG) Structured finance Non-U.S.
Financial guarantee obligations
Insured Financial Obligations Outstanding Principal Amount Excluding Loss Mitigation Bonds	$ 473,370,000,000	[1]	$ 518,772,000,000	[1]	$ 361,203,000,000	[1]	$ 387,929,000,000	[1]	$ 34,912,000,000	[1]	$ 37,540,000,000	[1]	$ 62,584,000,000	[1]	$ 74,535,000,000	[1]	$ 14,671,000,000	[1]	$ 18,768,000,000	[1]					$ 50,929,000,000	[1]	$ 61,564,000,000	[1]	$ 4,169,000,000	[1]	$ 4,502,000,000	[1]	$ 1,711,000,000	[1]	$ 1,706,000,000	[1]	$ 34,924,000,000	[1]	$ 42,187,000,000	[1]	$ 10,125,000,000	[1]	$ 13,169,000,000	[1]	$ 122,835,000,000	[1]	$ 135,665,000,000	[1]	$ 112,319,000,000	[1]	$ 124,525,000,000	[1]	$ 488,000,000	[1]	$ 875,000,000	[1]	$ 9,438,000,000	[1]	$ 9,543,000,000	[1]	$ 590,000,000	[1]	$ 722,000,000	[1]	$ 210,145,000,000	[1]	$ 225,984,000,000	[1]	$ 197,403,000,000	[1]	$ 210,124,000,000	[1]	$ 9,358,000,000	[1]	$ 9,781,000,000	[1]	$ 2,587,000,000	[1]	$ 4,670,000,000	[1]	$ 797,000,000	[1]	$ 1,409,000,000	[1]	$ 70,904,000,000	[1]	$ 73,262,000,000	[1]	$ 42,684,000,000	[1]	$ 44,213,000,000	[1]	$ 21,729,000,000	[1]	$ 22,885,000,000	[1]	$ 4,329,000,000	[1]	$ 3,737,000,000	[1]	$ 2,162,000,000	[1]	$ 2,427,000,000	[1]	$ 18,557,000,000	[1]	$ 22,297,000,000	[1]	$ 4,628,000,000	[1]	$ 4,565,000,000	[1]	$ 1,626,000,000	[1]	$ 2,293,000,000	[1]	$ 11,306,000,000	[1]	$ 14,398,000,000	[1]	$ 997,000,000	[1]	$ 1,041,000,000	[1]
Insured Financial Obligations Outstanding Principal Amount	474,667,000,000		519,893,000,000		361,237,000,000		387,967,000,000		34,912,000,000		37,540,000,000		63,847,000,000		75,618,000,000		14,671,000,000		18,768,000,000																																																																																																										1,297,000,000	1,121,000,000	34,000,000	38,000,000	0	0	1,263,000,000	1,083,000,000	0	0
% of Total Net Par Outstanding	100.00%	[1]	100.00%	[1]	100.00%	[1]	100.00%	[1]	100.00%	[1]	100.00%	[1]	100.00%	[1]	100.00%	[1]	100.00%	[1]	100.00%	[1]					10.80%	[1]	11.90%	[1]	1.20%	[1]	1.20%	[1]	4.90%	[1]	4.50%	[1]	55.80%	[1]	56.60%	[1]	69.00%	[1]	70.20%	[1]	25.90%	[1]	26.10%	[1]	31.10%	[1]	32.10%	[1]	1.40%	[1]	2.30%	[1]	15.10%	[1]	12.80%	[1]	4.00%	[1]	3.90%	[1]	44.40%	[1]	43.60%	[1]	54.60%	[1]	54.10%	[1]	26.80%	[1]	26.10%	[1]	4.10%	[1]	6.30%	[1]	5.50%	[1]	7.50%	[1]	15.00%	[1]	14.10%	[1]	11.80%	[1]	11.40%	[1]	62.20%	[1]	61.00%	[1]	6.90%	[1]	5.00%	[1]	14.70%	[1]	12.90%	[1]	3.90%	[1]	4.30%	[1]	1.30%	[1]	1.20%	[1]	4.70%	[1]	6.10%	[1]	18.10%	[1]	19.30%	[1]	6.80%	[1]	5.50%	[1]
Securities purchased to settle credit derivative contracts	218,000,000		220,000,000
Outstanding commitments to provide guaranties																					600,000,000	800,000,000
Expiration date for insured financial obligation commitments																							Oct. 01, 2013	Feb. 25, 2017
Outstanding commitments to provide guaranties, amount expiring prior to December 31, 2012																						$ 400,000,000

[1] In Third Quarter 2013, the Company adjusted its approach to assigning internal ratings. See "Refinement of Approach to Internal Credit Ratings and Surveillance Categories" below. This approach is reflected in the "Financial Guaranty Portfolio by Internal Rating" tables as of both September 30, 2013 and December 31, 2012.