Financial Guaranty Contracts Accounted for as Credit Derivatives - Net Fair Value and Expected Losses of Credit Derivatives by Sector (Details) - USD ($) $ in Millions		Dec. 31, 2015		Dec. 31, 2014
Credit Derivatives
Fair Value of Credit Derivative Asset (Liability), net	[1]	$ (365.0)	[2]	$ (895.0)
Expected Loss to be (Paid) Recovered	[3]	(16.0)		(58.0)
Pooled corporate obligations [Member]
Credit Derivatives
Fair Value of Credit Derivative Asset (Liability), net		(82.0)		(49.0)
Expected Loss to be (Paid) Recovered	[3]	(5.0)		(23.0)
Residential Mortgage-Backed Securities (RMBS) [Member] | United States [Member]
Credit Derivatives
Fair Value of Credit Derivative Asset (Liability), net		(98.0)		(494.0)
Expected Loss to be (Paid) Recovered	[3]	(38.0)		(73.0)
Commercial Mortgage Backed Securities (CMBS) [Member]
Credit Derivatives
Fair Value of Credit Derivative Asset (Liability), net		0.0		0.0
Expected Loss to be (Paid) Recovered	[3]	0.0		0.0
Other [Member]
Credit Derivatives
Fair Value of Credit Derivative Asset (Liability), net		(185.0)		(352.0)
Expected Loss to be (Paid) Recovered	[3]	27.0		38.0
Credit Risk Contract [Member]
Credit Derivatives
R&W Included in Credit Derivative Asset (Liability)		$ 0.4		$ 86.0

[1] December 31, 2015 amount includes $44 million of net fair value loss of credit derivatives acquired from Radian Asset.

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

[3] Includes R&W benefit of $0.4 million as of December 31, 2015 and $86 million as of December 31, 2014.