Financial Guarantee Insurance Contracts - Summary of Loss Reserve Roll-Forward, Net of Subrogation Recoverable and Reinsurance (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Feb. 12, 2018	Dec. 31, 2017	Dec. 31, 2016
Loss And Loss Adjustment Expense Reserves [Line Items]
Impact of VIE Consolidation on Gross Loss and Loss Reserves				$ (72)
Liability for Unpaid Claims and Claims Adjustment Expense, Foreign Currency Translation Gain (Loss)	$ (1)	$ (15)	$ 29
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	13	(224)	513
Schedule Of Insured Financial Obligations With Credit Deterioration Reductions Of Gross Claim Liability Rmbs Subrogation				(1,727)	$ (1,771)
Policyholder Benefits and Claims Incurred, Ceded	7	2	(20)
Net Incurred RMBS Subrogation Recoveries	(42)	(62)	(72)
satisfaction and discharge of segregated account deferred amounts						$ 3,000
satisfaction and discharge of segregated account deferred interest amounts						857
Loss and Loss Expense Benefit on Settled Deferred Amounts						$ 288
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Beginning gross loss and loss expense reserves	(107)	4,114	3,696
Less reinsurance on loss and loss expense reserves	23	41	31
Beginning balance of net loss and loss expense reserves	(130)	4,073	3,665
Current year:
Claim and loss expense payments, net of subrogation and reinsurance	0	0	(1)
Prior years:
Claim and loss expense (payments) recoveries, net of subrogation and reinsurance	(318)	(3,963)	(133)
Liability for Unpaid Claims and Claims Adjustment Expense, Net	(130)	4,073	3,665	(436)	(130)		$ 4,073	$ 3,665
Add reinsurance on loss and loss expense reserves	26	23	41
Ending gross loss and loss expense reserves	(482)	(107)	4,114
Balance Of Rmbs Subrogation Recoveries Net Of Reinsurance				(1,702)	(1,744)
Current Year Claims and Claims Adjustment Expense	1	5	6
Prior Year Claims and Claims Adjustment Expense	12	(228)	507
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid	$ 318	$ 3,964	$ 134
Random Samples [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Schedule Of Insured Financial Obligations With Credit Deterioration Reductions Of Gross Claim Liability Rmbs Subrogation				$ (1,727)	$ (1,771)		$ (1,834)	$ (1,907)