Financial Guaranty Contracts Accounted for as Credit Derivatives - Effect of Changes in Credit Spread (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015		Dec. 31, 2014
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Credit Risk Derivatives, 100 Percent Widening in Spreads, Effect on Fair Value	$ (1,590)	[1]
Credit Risk Derivatives, 50 Percent Widening in Spreads, Effect on Fair Value	(1,186)	[1]
Credit Risk Derivatives, 25 Percent Widening in Spreads, Effect on Fair Value	(986)	[1]
Credit Risk Derivatives, 10 Percent Widening in Spreads, Effect on Fair Value	(864)	[1]
Net fair value of credit derivatives	(782)	[1]	(895)
Credit Risk Derivatives, 10 Percent Narrowing in Spreads, Effect on Fair Value	(706)	[1]
Credit Risk Derivatives, 25 Percent Narrowing in Spreads, Effect on Fair Value	(593)	[1]
Credit Risk Derivatives, 50 Percent Narrowing in Spreads, Effect on Fair Value	(407)	[1]
Credit Risk Derivatives, 100 Percent Widening in Spreads, Effect on Unrealized Gain (Loss)	(808)	[1]
Credit Risk Derivatives, 50 Percent Widening in Spreads, Effect on Unrealized Gain (Loss)	(404)	[1]
Credit Risk Derivatives, 25 Percent Widening in Spreads, Effect on Unrealized Gain (Loss)	(204)	[1]
Credit Risk Derivatives, 10 Percent Widening in Spreads, Effect on Unrealized Gain (Loss)	(82)	[1]
Credit Risk Derivatives, Base Scenario, Effect on Unrealized Gain (Loss)	0	[1]
Credit Risk Derivatives, 10 Percent Narrowing in Spreads, Effect on Unrealized Gain (Loss)	76	[1]
Credit Risk Derivatives, 25 Percent Narrowing in Spreads, Effect on Unrealized Gain (Loss)	189	[1]
Credit Risk Derivatives, 50 Percent Narrowing in Spreads, Effect on Unrealized Gain (Loss)	$ 375	[1]

[1] Includes the effects of spreads on both the underlying asset classes and the Company’s own credit spread.