Financial Guarantee Insurance Contracts Financial Guarantee Insurance Contracts - Components of Loss and Loss Expense Reserves and Subrogation Recoverable (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Components of Loss and Loss Expense Reserves and Subrogation Recoverable [Line Items]
Ceded Loss And Loss Expenses Paid Not Yet Recovered	$ 1	$ 0	$ 0
Loss Reserves Ceded To Reinsurers	36	26	26	$ 23
Policyholder Benefits and Claims Incurred, Ceded	14	6
Liability for Unpaid Claims and Claims Adjustment Expense, Net	(429)	$ (514)	(508)	$ (130)
Loss and loss expense reserves	1,801		1,548
Subrogation recoverable	(2,194)		(2,029)
Claim liability reported on Balance Sheet, before reinsurance	(393)		(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	2,116		1,835
Subrogation recoverable	109		131
Claim liability reported on Balance Sheet, before reinsurance	2,225		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	(235)		(233)
Subrogation recoverable	(2,303)		(2,160)
Claim liability reported on Balance Sheet, before reinsurance	(2,538)		(2,394)
Unearned Premium Reserve [Member]
Components of Loss and Loss Expense Reserves and Subrogation Recoverable [Line Items]
Loss and loss expense reserves	(80)		(54)
Subrogation recoverable	0		0
Claim liability reported on Balance Sheet, before reinsurance	$ (80)		$ (54)