Financial Guaranty Contracts Accounted for as Credit Derivatives (Details 6) (USD $)	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
Credit Derivatives
Estimated net fair value (Pre-Tax)	$ (1,952,700,000)		$ (1,303,900,000)
Estimated Change in Gain/(Loss) (Pre-Tax)	(633,758,000)	(271,636,000)
100% widening in spreads
Credit Derivatives
Estimated net fair value (Pre-Tax)	(4,060,100,000)
Estimated Change in Gain/(Loss) (Pre-Tax)	(2,107,400,000)
50% widening in spreads
Credit Derivatives
Estimated net fair value (Pre-Tax)	(3,009,800,000)
Estimated Change in Gain/(Loss) (Pre-Tax)	(1,057,100,000)
25% widening in spreads
Credit Derivatives
Estimated net fair value (Pre-Tax)	(2,484,600,000)
Estimated Change in Gain/(Loss) (Pre-Tax)	(531,900,000)
10% widening in spreads
Credit Derivatives
Estimated net fair value (Pre-Tax)	(2,169,500,000)
Estimated Change in Gain/(Loss) (Pre-Tax)	(216,800,000)
Base Scenario
Credit Derivatives
Estimated net fair value (Pre-Tax)	(1,952,700,000)
10% narrowing in spreads
Credit Derivatives
Estimated net fair value (Pre-Tax)	(1,774,400,000)
Estimated Change in Gain/(Loss) (Pre-Tax)	178,300,000
25% narrowing in spreads
Credit Derivatives
Estimated net fair value (Pre-Tax)	(1,497,000,000)
Estimated Change in Gain/(Loss) (Pre-Tax)	455,700,000
50% narrowing in spreads
Credit Derivatives
Estimated net fair value (Pre-Tax)	(1,037,100,000)
Estimated Change in Gain/(Loss) (Pre-Tax)	$ 915,600,000