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, 2021	Jun. 30, 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	$ 1	$ 1	$ 1
Loss Reserves Ceded To Reinsurers	30	35	33	$ 26
Policyholder Benefits and Claims Incurred, Ceded	(2)	11
Liability for Unpaid Claims and Claims Adjustment Expense, Net	(486)	(437)	(430)	(508)
Subrogation recoverable	(2,097)		(2,156)
Claim liability reported on Balance Sheet, before reinsurance	(456)		(397)
Liability For Unpaid Claims And Claims Adjustment Expense (Gross of Subrogation Recoverable)	1,641		1,759
Liability for Claims and Claims Adjustment Expense	(456)	$ (401)	(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]
Subrogation recoverable	90		100
Present Value of Expected Net Cash Flows-Recoveries	(2,368)		(2,486)
Liability For Unpaid Claims And Claims Adjustment Expense (Gross of Subrogation Recoverable)	1,881		2,060
Liability for Claims and Claims Adjustment Expense	1,971		2,160
Present Value of Expected Net Cash Flows-Recoveries [Member]
Components of Loss and Loss Expense Reserves and Subrogation Recoverable [Line Items]
Subrogation recoverable	(2,188)		(2,256)
Liability For Unpaid Claims And Claims Adjustment Expense (Gross of Subrogation Recoverable)	(180)		(229)
Unearned Premium Reserve [Member]
Components of Loss and Loss Expense Reserves and Subrogation Recoverable [Line Items]
Subrogation recoverable	0		0
Liability For Unpaid Claims And Claims Adjustment Expense (Gross of Subrogation Recoverable)	(59)		(72)
Liability for Claims and Claims Adjustment Expense	$ (59)		$ (72)