Financial Guaranty Contracts Accounted for as Credit Derivatives (Details 6) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Credit Derivatives
Estimated net fair value (Pre-Tax)	$ (1,303,900,000)	$ (1,869,900,000)
Estimated Change in Gain/(Loss) (Pre-Tax)	553,779,000	(155,106,000)	(337,810,000)
100% widening in spreads
Credit Derivatives
Estimated net fair value (Pre-Tax)	(2,739,700,000)
Estimated Change in Gain/(Loss) (Pre-Tax)	(1,435,800,000)
50% widening in spreads
Credit Derivatives
Estimated net fair value (Pre-Tax)	(2,023,600,000)
Estimated Change in Gain/(Loss) (Pre-Tax)	(719,700,000)
25% widening in spreads
Credit Derivatives
Estimated net fair value (Pre-Tax)	(1,665,700,000)
Estimated Change in Gain/(Loss) (Pre-Tax)	(361,800,000)
10% widening in spreads
Credit Derivatives
Estimated net fair value (Pre-Tax)	(1,450,900,000)
Estimated Change in Gain/(Loss) (Pre-Tax)	(147,000,000)
Base Scenario
Credit Derivatives
Estimated net fair value (Pre-Tax)	(1,303,900,000)
10% narrowing in spreads
Credit Derivatives
Estimated net fair value (Pre-Tax)	(1,188,700,000)
Estimated Change in Gain/(Loss) (Pre-Tax)	115,200,000
25% narrowing in spreads
Credit Derivatives
Estimated net fair value (Pre-Tax)	(1,018,400,000)
Estimated Change in Gain/(Loss) (Pre-Tax)	285,500,000
50% narrowing in spreads
Credit Derivatives
Estimated net fair value (Pre-Tax)	(741,400,000)
Estimated Change in Gain/(Loss) (Pre-Tax)	$ 562,500,000