Contracts Accounted for as Insurance - Reconciliation of Net Expected Loss to be Paid and Expensed (Details) - USD ($) $ in Millions	Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Guarantor Obligations [Line Items]
Net expected loss to be paid (recovered) - financial guaranty insurance	$ 199	$ 466	$ 529	$ 471	$ 735	$ 737
Loss and LAE reserve - financial guaranty insurance contracts, net of reinsurance	(974)		$ (1,080)
Net expected loss to be expensed	347
Variable Interest Entity, Primary Beneficiary
Guarantor Obligations [Line Items]
Net expected loss to be expensed	28
Financial Guarantee Insurance And Other Product Line
Guarantor Obligations [Line Items]
Net expected loss to be paid (recovered) - financial guaranty insurance	(151)
Contra-paid, net	44
Salvage and subrogation recoverable, net, and other recoverable	1,120
Loss and LAE reserve - financial guaranty insurance contracts, net of reinsurance	(968)
Net expected loss to be expensed	$ 347