Investments and Cash - Internally Managed Investment Portfolio (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Debt and Equity Securities, FV-NI [Line Items]
Estimated Fair Value	$ 9,388	$ 9,624
Short-term investments at fair value	701	851
Other invested assets	211	214
Total investment portfolio	9,599	9,838
Externally managed
Debt and Equity Securities, FV-NI [Line Items]
Estimated Fair Value	7,224	7,301
Loss mitigation and other securities
Debt and Equity Securities, FV-NI [Line Items]
Estimated Fair Value	922	925
AssuredIM
Debt and Equity Securities, FV-NI [Line Items]
Estimated Fair Value	541	547
Equity method investments-AssuredIM Funds
Debt and Equity Securities, FV-NI [Line Items]
Other invested assets	71	91
Equity method investments-other
Debt and Equity Securities, FV-NI [Line Items]
Other invested assets	127	107
Other
Debt and Equity Securities, FV-NI [Line Items]
Other invested assets	$ 13	$ 16
BIG
Debt and Equity Securities, FV-NI [Line Items]
Fixed-maturity investments, non-investment grade, percent	8.20%	8.10%