Financial Guaranty Contracts Accounted for as Credit Derivatives - Distribution of Credit Derivative Net Par Outstanding by Internal Rating (Details) - USD ($) $ in Millions		9 Months Ended	12 Months Ended
		Sep. 30, 2016	Dec. 31, 2015
Credit Derivatives
Net Par Outstanding	[1]	$ 19,959.0	$ 25,594.0
BIG [Member]
Credit Derivatives
Net Par Outstanding		1,079.0	1,609.0
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member]
Credit Derivatives
Net Par Outstanding		$ 19,959.0	$ 25,594.0
Percentage of installment premiums denominated in currencies other than the U.S. dollar		100.00%	100.00%
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member] | Internal Credit Rating, AAA [Member]
Credit Derivatives
Net Par Outstanding		$ 10,778.0	$ 14,808.0
Percentage of installment premiums denominated in currencies other than the U.S. dollar		54.00%	57.90%
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member] | Internal Credit Rating, AA [Member]
Credit Derivatives
Net Par Outstanding		$ 4,025.0	$ 4,821.0
Percentage of installment premiums denominated in currencies other than the U.S. dollar		20.20%	18.80%
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member] | Internal Credit Rating, A [Member]
Credit Derivatives
Net Par Outstanding		$ 2,037.0	$ 2,144.0
Percentage of installment premiums denominated in currencies other than the U.S. dollar		10.20%	8.40%
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member] | Internal Credit Rating, BBB [Member]
Credit Derivatives
Net Par Outstanding		$ 2,040.0	$ 2,212.0
Percentage of installment premiums denominated in currencies other than the U.S. dollar		10.20%	8.60%
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member] | BIG [Member]
Credit Derivatives
Net Par Outstanding		$ 1,079.0	$ 1,609.0
Percentage of installment premiums denominated in currencies other than the U.S. dollar		5.40%	6.30%
CIFG Holding Inc. [Member]
Credit Derivatives
Net Par Outstanding		$ 2.3

[1] The September 30, 2016 total amount includes $2.3 million net par outstanding of credit derivatives acquired from CIFG.