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, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Loss And Loss Adjustment Expense Reserves [Line Items]
Impact of VIE Consolidation on Gross Loss and Loss Reserves	$ 464
Liability for Unpaid Claims and Claims Adjustment Expense, Foreign Currency Translation Gain (Loss)	(2)	$ 0	$ 2
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	(397)	(88)	225
Schedule Of Insured Financial Obligations With Credit Deterioration Reductions Of Gross Claim Liability Rmbs Subrogation	(140)	(1,730)
Policyholder Benefits and Claims Incurred, Ceded	41	(5)	7
Net Incurred RMBS Subrogation Recoveries	123	(20)	23
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Less reinsurance on loss and loss expense reserves	24
Beginning balance of net loss and loss expense reserves	(578)	(430)	(508)
Current year:
Claim and loss expense payments, net of subrogation and reinsurance	(7)	0	(1)
Prior years:
Claim and loss expense (payments) recoveries, net of subrogation and reinsurance	1,867	(59)	(148)
Liability for Unpaid Claims and Claims Adjustment Expense, Net	883	(578)	(430)	$ (508)
Add reinsurance on loss and loss expense reserves		24
Liabillity for Claim and Claim Adjustment Expense, with Impact of VIE Consolidation	534	(522)	(397)	(482)
Balance Of Rmbs Subrogation Recoveries Net Of Reinsurance	(140)	(1,704)
Current Year Claims and Claims Adjustment Expense	4	0	15
Prior Year Claims and Claims Adjustment Expense	(401)	(89)	210
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid	(1,860)	59	149
Ceded Loss And Loss Expenses Paid Not Yet Recovered		0	0
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	115	$ 55	33	$ 26
Liability For Unpaid Claims And Claims Adjustment Expense Net of VIE Consolidation			$ (430)
Settled Litigation
Prior years:
Claim and loss expense (payments) recoveries, net of subrogation and reinsurance	$ (1,687)