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		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Credit Derivatives
Realized gains on credit derivatives	$ 11	$ 14	$ 39	$ 52
Net credit derivative losses (paid and payable) recovered and recoverable and other settlements	4	(8)	8	(17)
Realized gains (losses) and other settlements	15	6	47	35
Net unrealized gains (losses):
Net change in unrealized gains (losses) on credit derivatives	6	80	(23)	265
Net change in fair value of credit derivatives	21	86	24	300
Pooled corporate obligations [Member]
Net unrealized gains (losses):
Net change in unrealized gains (losses) on credit derivatives	3	(24)	(37)	0
CMBS [Member]
Net unrealized gains (losses):
Net change in unrealized gains (losses) on credit derivatives	0	(3)	0	1
Other [Member]
Net unrealized gains (losses):
Net change in unrealized gains (losses) on credit derivatives	15	96	14	116
United States [Member] | RMBS [Member]
Net unrealized gains (losses):
Net change in unrealized gains (losses) on credit derivatives	(12)	11	0	148
Terminated And Settlement Of Credit Derivative Contracts [Member]
Credit Derivatives
Realized gains on credit derivatives	3	0	11	13
Net unrealized gains (losses):
Net change in unrealized gains (losses) on credit derivatives	11	99	81	98
Net par of terminated credit derivative contracts	$ 1,071	$ 405	$ 3,507	$ 969