Financial Guarantee Insurance Contracts - Summary of Loss Reserve Roll-Forward, Net of Subrogation Recoverable and Reinsurance (Detail) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 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	27	1
Prior Year Claims and Claims Adjustment Expense	90	12
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	117	12
Claim and loss expense payments, net of subrogation and reinsurance	0	0
Claim and loss expense (payments) recoveries, net of subrogation and reinsurance	(39)	(64)
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid	39	64
Liability for Unpaid Claims and Claims Adjustment Expense, Foreign Currency Translation Gain (Loss)	0	6
Liability for Unpaid Claims and Claims Adjustment Expense, Net	(508)	(130)	$ (130)	(430)	$ (508)	(176)
Add reinsurance on loss and loss expense reserves	35	26	23
Ending gross loss and loss expense reserves	(395)	(222)	(107)
Policyholder Benefits and Claims Incurred, Ceded	10	$ 5
Net Incurred RMBS Subrogation Recoveries	$ 36		$ (4)
Ceded Loss And Loss Expenses Paid Not Yet Recovered				$ 1	$ 0	$ 1