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, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
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)	$ 0	$ 2	(1)
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	(88)	225	13
Schedule Of Insured Financial Obligations With Credit Deterioration Reductions Of Gross Claim Liability Rmbs Subrogation	(1,730)	(1,751)
Policyholder Benefits and Claims Incurred, Ceded	(5)	11	7
Net Incurred RMBS Subrogation Recoveries	(20)	23	(42)
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Less reinsurance on loss and loss expense reserves	33
Beginning balance of net loss and loss expense reserves	(430)	(436)	(130)
Current year:
Claim and loss expense payments, net of subrogation and reinsurance	0	(1)	0
Prior years:
Claim and loss expense (payments) recoveries, net of subrogation and reinsurance	(59)	(148)	(318)
Liability for Unpaid Claims and Claims Adjustment Expense, Net	(577)	(430)	(436)	$ (130)
Add reinsurance on loss and loss expense reserves		33
Liabillity for Claim and Claim Adjustment Expense, with Impact of VIE Consolidation	(522)	(397)	(482)
Balance Of Rmbs Subrogation Recoveries Net Of Reinsurance	(1,704)	(1,725)
Current Year Claims and Claims Adjustment Expense	0	15	1
Prior Year Claims and Claims Adjustment Expense	(89)	210	12
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid	59	149	318
Ceded Loss And Loss Expenses Paid Not Yet Recovered		1	0
Liability for Claims and Claims Adjustment Expense	1,570	1,759	(482)	(107)
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	$ 55	33	26	$ 23
Liability For Unpaid Claims And Claims Adjustment Expense Net of VIE Consolidation			$ (508)
Liability for Claims and Claims Adjustment Expense Net of Reinsurance Recoverables		$ (397)