Financial Guarantee Insurance Contracts Financial Guarantee Insurance Contracts - Components of Loss and Loss Expense Reserves and Subrogation Recoverable (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Components of Loss and Loss Expense Reserves and Subrogation Recoverable [Line Items]
Ceded Loss And Loss Expenses Paid Not Yet Recovered	$ 0	$ 1	$ 1
Loss Reserves Ceded To Reinsurers	33	35	33	$ 26
Policyholder Benefits and Claims Incurred, Ceded	1	10
Liability for Unpaid Claims and Claims Adjustment Expense, Net	(447)	$ (430)	(430)	$ (508)
Loss and loss expense reserves	1,662		1,759
Subrogation recoverable	(2,076)		(2,156)
Claim liability reported on Balance Sheet, before reinsurance	(414)		(397)
Present Value of Expected Net Cash Flows- Claims and Loss Expenses [Member]
Components of Loss and Loss Expense Reserves and Subrogation Recoverable [Line Items]
Loss and loss expense reserves	1,831		2,060
Subrogation recoverable	99		100
Claim liability reported on Balance Sheet, before reinsurance	1,930		2,160
Present Value of Expected Net Cash Flows-Recoveries [Member]
Components of Loss and Loss Expense Reserves and Subrogation Recoverable [Line Items]
Loss and loss expense reserves	(102)		(229)
Subrogation recoverable	(2,175)		(2,256)
Claim liability reported on Balance Sheet, before reinsurance	(2,277)		(2,486)
Unearned Premium Reserve [Member]
Components of Loss and Loss Expense Reserves and Subrogation Recoverable [Line Items]
Loss and loss expense reserves	(67)		(72)
Subrogation recoverable	0		0
Claim liability reported on Balance Sheet, before reinsurance	$ (67)		$ (72)