Contracts Accounted for as Insurance - Reconciliation of Net Expected Loss to be Paid and Expensed (Details) - USD ($) $ in Millions	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Guarantor Obligations [Line Items]
Net expected loss to be paid (recovered) - financial guaranty insurance	$ (466)	$ (472)	$ (529)	$ (735)	$ (660)	$ (737)
Loss and LAE reserve - financial guaranty insurance contracts, net of reinsurance	(1,058)		$ (1,080)
Net expected loss to be expensed	359
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	410
Contra-paid, net	40
Salvage and subrogation recoverable, net, and other recoverable	964
Loss and LAE reserve - financial guaranty insurance contracts, net of reinsurance	(1,055)
Net expected loss to be expensed	$ 359