Investments and Cash - Investment Portfolio Carrying Value (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Schedule of Cost-method Investments [Line Items]
Short-term investments, at fair value	$ 1,225	$ 851
Other invested assets	181	214
Total investments	$ 9,608	$ 9,838
BIG
Schedule of Cost-method Investments [Line Items]
Fixed-maturity securities rated as BIG	7.50%	8.10%
Externally managed
Schedule of Cost-method Investments [Line Items]
Estimated Fair Value	$ 6,843	$ 7,301
Loss mitigation securities and other
Schedule of Cost-method Investments [Line Items]
Estimated Fair Value	818	925
AssuredIM managed
Schedule of Cost-method Investments [Line Items]
Estimated Fair Value	541	547
AssuredIM Fund
Schedule of Cost-method Investments [Line Items]
Other invested assets	0	91
Equity method investments - other
Schedule of Cost-method Investments [Line Items]
Other invested assets	169	107
Other
Schedule of Cost-method Investments [Line Items]
Other invested assets	$ 12	$ 16