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, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Feb. 12, 2018
Loss And Loss Adjustment Expense Reserves [Line Items]
Impact of VIE Consolidation on Gross Loss and Loss Reserves				$ 0
Liability for Unpaid Claims and Claims Adjustment Expense, Foreign Currency Translation Gain (Loss)	$ 2	$ (1)	$ (15)
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	225	13	(224)
Schedule Of Insured Financial Obligations With Credit Deterioration Reductions Of Gross Claim Liability Rmbs Subrogation				(1,751)	$ (1,727)
Policyholder Benefits and Claims Incurred, Ceded	11	7	2
Net Incurred RMBS Subrogation Recoveries	23	(42)	(62)
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	(482)	(107)	4,114
Less reinsurance on loss and loss expense reserves	26	23	41
Beginning balance of net loss and loss expense reserves	(436)	(130)	4,073
Current year:
Claim and loss expense payments, net of subrogation and reinsurance	(1)	0	0
Prior years:
Claim and loss expense (payments) recoveries, net of subrogation and reinsurance	(148)	(318)	(3,963)
Liability for Unpaid Claims and Claims Adjustment Expense, Net	(436)	(130)	4,073	(430)	(436)	$ (130)
Add reinsurance on loss and loss expense reserves	33	26	23
us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseNet					(508)
Ending gross loss and loss expense reserves	(397)	(482)	(107)
Balance Of Rmbs Subrogation Recoveries Net Of Reinsurance				(1,725)	(1,702)
Current Year Claims and Claims Adjustment Expense	15	1	5
Prior Year Claims and Claims Adjustment Expense	210	12	(228)
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid	$ 149	$ 318	$ 3,964
Ceded Loss And Loss Expenses Paid Not Yet Recovered				$ 1	$ 0	$ 1