Financial Guaranty Contracts Accounted for as Credit Derivatives - CDS Spread and Components of Credit Derivative Assets (Liabilities) (Details) - USD ($) $ in Millions	Sep. 30, 2016		Jun. 30, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Dec. 31, 2014
Credit Derivatives
Fair value of credit derivatives before effect of AGC and AGM credit spreads	$ (1,020)			$ (1,448)
Plus: Effect of AGC and AGM credit spreads	539			1,083
Net fair value of credit derivatives	$ (481)	[1]		$ (365)
Credit Risk Contract, 5 Year Spread [Member] | AGC [Member]
Credit Derivatives
Quoted price of CDS contract (as a percent)	1.70%		2.65%	3.76%	3.31%	3.90%	3.23%
Credit Risk Contract, 5 Year Spread [Member] | AGM [Member]
Credit Derivatives
Quoted price of CDS contract (as a percent)	1.70%		2.65%	3.66%	3.37%	4.10%	3.25%
Credit Risk Contract, 1 Year Spread [Member] | AGC [Member]
Credit Derivatives
Quoted price of CDS contract (as a percent)	0.31%		0.45%	1.39%	1.12%	1.20%	0.80%
Credit Risk Contract, 1 Year Spread [Member] | AGM [Member]
Credit Derivatives
Quoted price of CDS contract (as a percent)	0.31%		0.47%	1.31%	1.04%	1.25%	0.85%

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