Financial Guarantee Insurance Contracts Financial Guarantee Insurance Contracts - Components of Loss and Loss Expense Reserves and Subrogation Recoverable (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Components of Loss and Loss Expense Reserves and Subrogation Recoverable [Line Items]
Policyholder Benefits and Claims Incurred, Ceded	$ 11	$ 7	$ 2
Liability for Unpaid Claims and Claims Adjustment Expense, Net	(430)	(436)	$ (130)	$ 4,073
Loss and loss expense reserves	1,759	1,548
Subrogation recoverable	(2,156)	(2,029)
Claim liability reported on Balance Sheet, before reinsurance	(397)	(482)
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	2,060	1,835
Subrogation recoverable	100	131
Claim liability reported on Balance Sheet, before reinsurance	2,160	1,966
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	(229)	(233)
Subrogation recoverable	(2,256)	(2,160)
Claim liability reported on Balance Sheet, before reinsurance	(2,486)	(2,394)
Unearned Premium Reserve [Member]
Components of Loss and Loss Expense Reserves and Subrogation Recoverable [Line Items]
Loss and loss expense reserves	(72)	(54)
Subrogation recoverable	0	0
Claim liability reported on Balance Sheet, before reinsurance	$ (72)	$ (54)