Financial Guaranty Contracts Accounted for as Credit Derivatives - Narrative (Details)	3 Months Ended	6 Months Ended		12 Months Ended
	Jun. 30, 2016 USD ($) Transaction	Jun. 30, 2016 USD ($) Transaction	Jun. 30, 2015 USD ($)	Dec. 31, 2015 USD ($)	May 31, 2016 USD ($)	Mar. 31, 2016 USD ($)
Credit Derivatives
Estimated remaining weighted average life of credit derivatives (in years)		5 years		5 years 4 months 24 days
Net par outstanding	$ 20,014,000,000	$ 20,014,000,000		$ 25,594,000,000
Gain due to change in methodology in expected recoveries			$ 49,000,000
Collateral agreed to be posted	$ 216,000,000	$ 216,000,000		305,000,000
Market value collateralized debt obligations of corporate obligations [Member]
Credit Derivatives
Average obligor size (less than)	1.00%	1.00%
Maximum exposure of one industry (as a percent)	10.00%	10.00%
Net par outstanding	$ 12,653,000,000	$ 12,653,000,000		17,523,000,000
Other pooled infrastructure [Member]
Credit Derivatives
Net par outstanding	$ 1,600,000,000	$ 1,600,000,000
Pooled infrastructure [Member]
Credit Derivatives
Number of transactions | Transaction	1	1
Remaining other CDS [Member]
Credit Derivatives
Net par outstanding	$ 4,100,000,000	$ 4,100,000,000
CMBS [Member]
Credit Derivatives
Net par outstanding	384,000,000	384,000,000		530,000,000
Collateral Debt Obligations, Collateral Requirement [Member]
Credit Derivatives
Amount of par subject to collateral for which the amount of collateral is capped	1,000,000,000	1,000,000,000				$ 3,700,000,000
Derivative, Collateral Posting Requirement					$ 0
Derivative, Reduction Of Notional Amount	2,700,000,000
Collateral Debt Obligations, Collateral Cap Negotiated [Member]
Credit Derivatives
Amount of par subject to collateral for which the amount of collateral is capped	800,000,000	800,000,000
Collateral Debt Obligations, No Cap Negotiated [Member]
Credit Derivatives
Amount of par subject to collateral for which the amount of collateral is capped	500,000,000	500,000,000				$ 575,000,000
Notional amount subject to collateral based on movements in the mark-to-market valuation of the underlying exposure	208,000,000	208,000,000		221,000,000
Collateral agreed to be posted	216,000,000	216,000,000		305,000,000
Collateral posted, based on mark-to-market valuation	$ 17,000,000	$ 17,000,000		$ 23,000,000
Credit Risk Contract [Member]
Credit Derivatives
Number of terminated transactions | Transaction	2	2