Financial Guarantee Insurance Contracts Financial Guarantee Insurance Contracts - Components of Loss and Loss Expense Reserves and Subrogation Recoverable (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2020	Jun. 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	35	27	26	$ 23
Policyholder Benefits and Claims Incurred, Ceded	11	6
Liability for Unpaid Claims and Claims Adjustment Expense, Net	(437)	$ (446)	(508)	$ (130)
Loss and loss expense reserves	1,814		1,548
Subrogation recoverable	(2,215)		(2,029)
Claim liability reported on Balance Sheet, before reinsurance	(401)		(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	2,134		1,835
Subrogation recoverable	120		131
Claim liability reported on Balance Sheet, before reinsurance	2,254		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	(238)		(233)
Subrogation recoverable	(2,336)		(2,160)
Claim liability reported on Balance Sheet, before reinsurance	(2,573)		(2,394)
Unearned Premium Reserve [Member]
Components of Loss and Loss Expense Reserves and Subrogation Recoverable [Line Items]
Loss and loss expense reserves	(82)		(54)
Subrogation recoverable	0		0
Claim liability reported on Balance Sheet, before reinsurance	$ (82)		$ (54)