Variable Interest Entities - Unpaid Principal (Details) - Variable Interest Entity, Primary Beneficiary - Financial Guaranty Variable Interest Entities - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Variable Interest Entity [Line Items]
FG VIEs’ assets	$ 274	$ 274
FG VIEs’ liabilities with recourse	18	15
FG VIEs’ liabilities without recourse	16	16
Unpaid principal balance for FG VIEs’ assets that were 90 days or more past due	69	68
Unpaid principal for FG VIEs’ liabilities with recourse	$ 319	$ 330