Financial Guarantee Insurance Contracts - Summary of Loss Reserve Roll-Forward, Net of Subrogation Recoverable and Reinsurance (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Beginning gross loss and loss expense reserves	$ (397)	$ (482)
Less reinsurance on loss and loss expense reserves	33	26
Beginning balance of net loss and loss expense reserves	(430)	(508)
Current Year Claims and Claims Adjustment Expense	0	16
Prior Year Claims and Claims Adjustment Expense	(18)	116
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	(18)	132
Claim and loss expense payments, net of subrogation and reinsurance	0	0
Claim and loss expense (payments) recoveries, net of subrogation and reinsurance	(38)	(61)
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid	38	61
Liability for Unpaid Claims and Claims Adjustment Expense, Foreign Currency Translation Gain (Loss)	0	0
Liability for Unpaid Claims and Claims Adjustment Expense, Net	(486)	(437)	$ (430)
Add reinsurance on loss and loss expense reserves	30	35
Ending gross loss and loss expense reserves	(456)	(401)
Policyholder Benefits and Claims Incurred, Ceded	(2)	11
Net Incurred RMBS Subrogation Recoveries	(10)	30
Ceded Loss And Loss Expenses Paid Not Yet Recovered	$ 1	$ 1	$ 1