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, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Credit Derivatives
Realized gains on credit derivatives	$ 18	$ 15	$ 28	$ 38
Net credit derivative losses (paid and payable) recovered and recoverable and other settlements	6	(7)	4	(9)
Realized gains (losses) and other settlements	24	8	32	29
Net unrealized gains (losses):
Net change in unrealized gains (losses) on credit derivatives	39	82	(29)	185
Net change in fair value of credit derivatives	63	90	3	214
Pooled corporate obligations [Member]
Net unrealized gains (losses):
Net change in unrealized gains (losses) on credit derivatives	8	7	(40)	24
CMBS [Member]
Net unrealized gains (losses):
Net change in unrealized gains (losses) on credit derivatives	0	4	0	4
Other [Member]
Net unrealized gains (losses):
Net change in unrealized gains (losses) on credit derivatives	4	9	(1)	20
United States [Member] | RMBS [Member]
Net unrealized gains (losses):
Net change in unrealized gains (losses) on credit derivatives	27	62	12	137
Credit Risk Contract [Member]
Net unrealized gains (losses):
Net change in unrealized gains (losses) on credit derivatives	59	0	70	(1)
Net change in fair value of credit derivatives	8	2	8	13
Net par of terminated credit derivative contracts	$ 2,436	$ 471	$ 2,436	$ 564