Financial Guaranty Contracts Accounted for as Credit Derivatives - Net Change in Fair Value of Credit Derivatives Gains (Losses) (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Credit Derivatives
Realized gains on credit derivatives	[1]	$ 15.0	$ 21.0	$ 38.0	$ 41.0
Net credit derivative losses (paid and payable) recovered and recoverable and other settlements		(7.0)	(6.0)	(9.0)	(7.0)
Realized gains (losses) and other settlements on credit derivatives		8.0	15.0	29.0	34.0
Net change in unrealized gains (losses) on credit derivatives		82.0	88.0	185.0	(142.0)
Net change in fair value of credit derivatives		90.0	103.0	214.0	(108.0)
Pooled corporate obligations [Member]
Credit Derivatives
Net change in unrealized gains (losses) on credit derivatives		7.0	64.0	24.0	6.0
CMBS [Member]
Credit Derivatives
Net change in unrealized gains (losses) on credit derivatives		4.0	2.0	4.0	2.0
Other [Member]
Credit Derivatives
Net change in unrealized gains (losses) on credit derivatives		9.0	17.0	20.0	(15.0)
United States [Member] | RMBS [Member]
Credit Derivatives
Net change in unrealized gains (losses) on credit derivatives		62.0	5.0	137.0	(135.0)
Credit Risk Contract [Member]
Credit Derivatives
Realized gains (losses) and other settlements		1.8	0.5	12.6	0.7
Net par of terminated CDS contracts		$ 500.0	$ 200.0	$ 600.0	$ 1,300.0

[1] Includes realized gain due to terminations of CDS contracts of $1.8 million and $0.5 million for Second Quarter 2015 and Second Quarter 2014, respectively, and $12.6 million and $0.7 million for Six Months 2015 and Six Months 2014, respectively. Net par of $0.5 billion and $0.2 billion were terminated in Second Quarter 2015 and Second Quarter 2014, respectively, and $0.6 billion and $1.3 billion for Six Months 2015 and Six Months 2014, respectively. CDS terminations in Six Months 2015 also included a payment received from the resolution of a dispute related to a termination of CDS in 2008.