Financial Guaranty Contracts Accounted for as Credit Derivatives (Details 5) (USD $)	3 Months Ended
	Mar. 31, 2013	Jan. 17, 2013	Dec. 31, 2012	Mar. 31, 2013 5 year spread [Member] AGC	Dec. 31, 2012 5 year spread [Member] AGC	Dec. 31, 2011 5 year spread [Member] AGC	Mar. 31, 2013 5 year spread [Member] AGM	Dec. 31, 2012 5 year spread [Member] AGM	Dec. 31, 2011 5 year spread [Member] AGM	Mar. 31, 2013 1 year spread [Member] [Member] AGC	Dec. 31, 2012 1 year spread [Member] [Member] AGC	Dec. 31, 2011 1 year spread [Member] [Member] AGC	Mar. 31, 2013 1 year spread [Member] [Member] AGM	Dec. 31, 2012 1 year spread [Member] [Member] AGM	Dec. 31, 2011 1 year spread [Member] [Member] AGM
Components of Credit Derivative Assets (Liabilities)
Quoted price of CDS contract (as a percent)				3.97%	6.78%	11.40%	3.80%	5.36%	7.78%	0.59%	2.70%	9.65%	0.60%	2.57%	5.38%
Credit derivative assets	$ 125,000,000		$ 141,000,000
Credit derivative liabilities	(2,518,000,000)		(1,934,000,000)
Net fair value of credit derivatives	(2,393,000,000)		(1,793,000,000)
Fair value of credit derivatives before effect of AGC and AGM credit spreads
Fair value of credit derivatives before effect of AGC and AGM credit spreads	(4,296,000,000)		(4,809,000,000)
Plus: Effect of AGC and AGM credit spreads	1,903,000,000		3,016,000,000
Other credit derivative disclosures
Gain from tightening of spreads	513,000,000
Par insured that could be terminated if ratings were downgraded	2,000,000,000
Number of CDS Counterparites that would constitute a termination event	1
Amount of par subject to collateral posting	12,300,000,000
Collateral agreed to be posted		709,000,000
Credit Derivative, Collateral Posted, based in MTM valuation		64,000,000
Credit Derivative, Notional Amount Subjest to Collateral based in MTM valuation	400,000,000
Amount of par subject to collateral for which the amount of collateral is capped	675,000,000
Cap on collateral requirement at current ratings levels	$ 11,900,000,000