Financial Guarantee Insurance Contracts - Summary of Loss Reserve Roll-Forward, Net of Subrogation Recoverable and Reinsurance (Detail) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019
Loss And Loss Adjustment Expense Reserves [Line Items]
Impact of VIE Consolidation on Gross Loss and Loss Reserves			$ 0		$ (72)
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Beginning gross loss and loss expense reserves	$ (482)	$ (107)
Less reinsurance on loss and loss expense reserves	26	23
Beginning balance of net loss and loss expense reserves	(508)	(130)
Current Year Claims and Claims Adjustment Expense	16	1
Prior Year Claims and Claims Adjustment Expense	116	(122)
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	132	(121)
Claim and loss expense payments, net of subrogation and reinsurance	0	0
Claim and loss expense (payments) recoveries, net of subrogation and reinsurance	(61)	(194)
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid	61	194
Liability for Unpaid Claims and Claims Adjustment Expense, Foreign Currency Translation Gain (Loss)	0	(1)
Liability for Unpaid Claims and Claims Adjustment Expense, Net	(508)	(130)	(437)	$ (508)	(446)
Add reinsurance on loss and loss expense reserves	35	27
Ending gross loss and loss expense reserves	(401)	(491)
Policyholder Benefits and Claims Incurred, Ceded	11	6
Net Incurred RMBS Subrogation Recoveries	$ 30	$ (8)
Ceded Loss And Loss Expenses Paid Not Yet Recovered			$ 1	$ 0	$ 0