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, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Components of Loss and Loss Expense Reserves and Subrogation Recoverable [Line Items]
Policyholder Benefits and Claims Incurred, Ceded	$ 7	$ 2	$ (20)
Liability for Unpaid Claims and Claims Adjustment Expense, Net	(436)	(130)	$ 4,073	$ 3,665
Loss and loss expense reserves	1,548	1,826
Subrogation recoverable	(2,029)	(1,933)
Claim liability reported on Balance Sheet, before reinsurance	(482)	(107)
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	1,835	2,246
Subrogation recoverable	131	176
Claim liability reported on Balance Sheet, before reinsurance	1,966	2,422
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	(233)	(314)
Subrogation recoverable	(2,160)	(2,109)
Claim liability reported on Balance Sheet, before reinsurance	(2,394)	(2,422)
Unearned Premium Reserve [Member]
Components of Loss and Loss Expense Reserves and Subrogation Recoverable [Line Items]
Loss and loss expense reserves	(54)	(107)
Subrogation recoverable	0	0
Claim liability reported on Balance Sheet, before reinsurance	$ (54)	$ (107)