Financial Guaranty Contracts Accounted for as Credit Derivatives - Narrative (Details) $ in Millions		3 Months Ended				9 Months Ended			12 Months Ended
		Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($) Transaction		Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) Transaction		Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)	Oct. 09, 2015 Transaction
Credit Derivatives
Estimated remaining weighted average life of credit derivatives (in years)						5 years 3 months 23 days			4 years 7 months 30 days
Net par outstanding			$ 29,633.0	[1]		$ 29,633.0	[1]		$ 34,996.0
Gain due to change in methodology in expected recoveries						49.0
Gain from narrowing of spreads						56.0
Collateral agreed to be posted			$ 333.0			$ 333.0			376.0
Market value collateralized debt obligations of corporate obligations [Member]
Credit Derivatives
Maximum average obligor size (as a percent)			1.00%			1.00%
Maximum exposure of one industry (as a percent)			10.00%			10.00%
Net par outstanding			$ 19,135.0			$ 19,135.0			23,621.0
Other pooled infrastructure [Member]
Credit Derivatives
Net par outstanding			$ 1,900.0			$ 1,900.0
Pooled infrastructure [Member]
Credit Derivatives
Number of transactions | Transaction			1			1
Remaining other CDS [Member]
Credit Derivatives
Net par outstanding			$ 4,900.0			$ 4,900.0
CMBS [Member]
Credit Derivatives
Net par outstanding			1,127.0			1,127.0			1,952.0
Collateral Debt Obligations, Collateral Requirement [Member]
Credit Derivatives
Amount of par subject to collateral for which the amount of collateral is capped			4,900.0			4,900.0
Collateral Debt Obligations, Collateral Cap Negotiated [Member]
Credit Derivatives
Amount of par subject to collateral for which the amount of collateral is capped			4,700.0			4,700.0
Collateral Debt Obligations, No Cap Negotiated [Member]
Credit Derivatives
Amount of par subject to collateral for which the amount of collateral is capped			575.0			575.0
Collateral agreed to be posted			333.0			333.0			376.0
Notional amount subject to collateral based on movements in the mark-to-market valuation of the underlying exposure			235.0			235.0			242.0
Collateral posted, based on mark-to-market valuation			20.0			20.0			$ 25.0
Credit Risk Contract [Member]
Credit Derivatives
Realized gain due to termination of CDS contracts			0.3		$ (0.1)	12.9		$ 0.6
Radian [Member]
Credit Derivatives
Net par outstanding	[1]		$ 4,300.0			$ 4,300.0
Scenario, Forecast [Member] | Credit Risk Contract [Member] | CMBS [Member]
Credit Derivatives
Realized gain due to termination of CDS contracts		$ 34.0
First Lien [Member] | Scenario, Forecast [Member] | Credit Risk Contract [Member] | RMBS [Member]
Credit Derivatives
Realized gain due to termination of CDS contracts		$ 293.0
First Lien [Member] | Subsequent Event [Member] | Credit Risk Contract [Member] | RMBS [Member]
Credit Derivatives
Number of transactions terminated | Transaction										5

[1] The September 30, 2015 total amount includes $4.3 billion net par outstanding of credit derivatives acquired from Radian Asset.