Fair Value Measurement - Financial Instruments Carried at Fair Value (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Assets:
Estimated Fair Value	$ 6,091	$ 6,307
Fixed-maturity securities, trading	272	318
Short-term investments, at fair value	1,649	1,661
Other invested assets	875	829
FG VIEs’ assets	167	174
Other assets	123	123
Liabilities:
Credit derivative liabilities	43	53
FG VIEs' liabilities	399	554
Obligations of state and political subdivisions
Assets:
Estimated Fair Value	2,331	2,661
U.S. government and agencies
Assets:
Estimated Fair Value	55	60
Corporate securities
Assets:
Estimated Fair Value	2,101	2,141
RMBS
Assets:
Estimated Fair Value	402	342
CMBS
Assets:
Estimated Fair Value	196	151
Non-U.S. government securities
Assets:
Estimated Fair Value	84	100
Recurring
Assets:
Short-term investments, at fair value	1,649	1,661
Other invested assets	4	3
Other assets	123	123
Total assets carried at fair value	8,659	8,917
Liabilities:
Credit derivative liabilities	43	53
Total liabilities carried at fair value	442	607
Recurring | Financial Guaranty Variable Interest Entities
Liabilities:
FG VIEs' liabilities	399	554
Recurring | Financial Guaranty Variable Interest Entities
Assets:
FG VIEs’ assets	167	174
Recurring | Consolidated Investment Vehicles
Assets:
FG VIEs’ assets	353	331
Recurring | Fixed-maturity securities, available-for-sale
Assets:
Estimated Fair Value	6,091	6,307
Fixed-maturity securities, trading	272	318
Recurring | Fixed-maturity securities, available-for-sale | Obligations of state and political subdivisions
Assets:
Estimated Fair Value	2,331	2,661
Recurring | Fixed-maturity securities, available-for-sale | U.S. government and agencies
Assets:
Estimated Fair Value	55	60
Recurring | Fixed-maturity securities, available-for-sale | Corporate securities
Assets:
Estimated Fair Value	2,101	2,141
Recurring | Fixed-maturity securities, available-for-sale | RMBS
Assets:
Estimated Fair Value	402	342
Recurring | Fixed-maturity securities, available-for-sale | CMBS
Assets:
Estimated Fair Value	196	151
Recurring | Fixed-maturity securities, available-for-sale | Asset-backed securities
Assets:
Estimated Fair Value	922	852
Recurring | Fixed-maturity securities, available-for-sale | Non-U.S. government securities
Assets:
Estimated Fair Value	84	100
Recurring | Fund Investments | Equity securities and warrants | Consolidated Investment Vehicles
Assets:
FG VIEs’ assets	89	83
Recurring | Fund Investments | Structured products | Consolidated Investment Vehicles
Assets:
FG VIEs’ assets	264	248
Recurring | Level 1
Assets:
Short-term investments, at fair value	1,649	1,657
Other invested assets	0	0
Other assets	61	55
Total assets carried at fair value	1,710	1,712
Liabilities:
Credit derivative liabilities	0	0
Total liabilities carried at fair value	0	0
Recurring | Level 1 | Financial Guaranty Variable Interest Entities
Liabilities:
FG VIEs' liabilities	0	0
Recurring | Level 1 | Financial Guaranty Variable Interest Entities
Assets:
FG VIEs’ assets	0	0
Recurring | Level 1 | Consolidated Investment Vehicles
Assets:
FG VIEs’ assets	0	0
Recurring | Level 1 | Fixed-maturity securities, available-for-sale
Assets:
Estimated Fair Value	0	0
Fixed-maturity securities, trading	0	0
Recurring | Level 1 | Fixed-maturity securities, available-for-sale | Obligations of state and political subdivisions
Assets:
Estimated Fair Value	0	0
Recurring | Level 1 | Fixed-maturity securities, available-for-sale | U.S. government and agencies
Assets:
Estimated Fair Value	0	0
Recurring | Level 1 | Fixed-maturity securities, available-for-sale | Corporate securities
Assets:
Estimated Fair Value	0	0
Recurring | Level 1 | Fixed-maturity securities, available-for-sale | RMBS
Assets:
Estimated Fair Value	0	0
Recurring | Level 1 | Fixed-maturity securities, available-for-sale | CMBS
Assets:
Estimated Fair Value	0	0
Recurring | Level 1 | Fixed-maturity securities, available-for-sale | Asset-backed securities
Assets:
Estimated Fair Value	0	0
Recurring | Level 1 | Fixed-maturity securities, available-for-sale | Non-U.S. government securities
Assets:
Estimated Fair Value	0	0
Recurring | Level 1 | Fund Investments | Equity securities and warrants | Consolidated Investment Vehicles
Assets:
FG VIEs’ assets	0	0
Recurring | Level 1 | Fund Investments | Structured products | Consolidated Investment Vehicles
Assets:
FG VIEs’ assets	0	0
Recurring | Level 2
Assets:
Short-term investments, at fair value	0	4
Other invested assets	0	0
Other assets	55	52
Total assets carried at fair value	5,482	5,780
Liabilities:
Credit derivative liabilities	0	0
Total liabilities carried at fair value	0	0
Recurring | Level 2 | Financial Guaranty Variable Interest Entities
Liabilities:
FG VIEs' liabilities	0	0
Recurring | Level 2 | Financial Guaranty Variable Interest Entities
Assets:
FG VIEs’ assets	0	0
Recurring | Level 2 | Consolidated Investment Vehicles
Assets:
FG VIEs’ assets	44	62
Recurring | Level 2 | Fixed-maturity securities, available-for-sale
Assets:
Estimated Fair Value	5,111	5,344
Fixed-maturity securities, trading	272	318
Recurring | Level 2 | Fixed-maturity securities, available-for-sale | Obligations of state and political subdivisions
Assets:
Estimated Fair Value	2,325	2,655
Recurring | Level 2 | Fixed-maturity securities, available-for-sale | U.S. government and agencies
Assets:
Estimated Fair Value	55	60
Recurring | Level 2 | Fixed-maturity securities, available-for-sale | Corporate securities
Assets:
Estimated Fair Value	2,101	2,141
Recurring | Level 2 | Fixed-maturity securities, available-for-sale | RMBS
Assets:
Estimated Fair Value	249	188
Recurring | Level 2 | Fixed-maturity securities, available-for-sale | CMBS
Assets:
Estimated Fair Value	196	151
Recurring | Level 2 | Fixed-maturity securities, available-for-sale | Asset-backed securities
Assets:
Estimated Fair Value	101	49
Recurring | Level 2 | Fixed-maturity securities, available-for-sale | Non-U.S. government securities
Assets:
Estimated Fair Value	84	100
Recurring | Level 2 | Fund Investments | Equity securities and warrants | Consolidated Investment Vehicles
Assets:
FG VIEs’ assets	0	3
Recurring | Level 2 | Fund Investments | Structured products | Consolidated Investment Vehicles
Assets:
FG VIEs’ assets	44	59
Recurring | Level 3
Assets:
Short-term investments, at fair value	0	0
Other invested assets	4	3
Other assets	7	16
Total assets carried at fair value	1,467	1,425
Liabilities:
Credit derivative liabilities	43	53
Total liabilities carried at fair value	442	607
Recurring | Level 3 | Financial Guaranty Variable Interest Entities
Liabilities:
FG VIEs' liabilities	399	554
Recurring | Level 3 | Financial Guaranty Variable Interest Entities
Assets:
FG VIEs’ assets	167	174
Recurring | Level 3 | Consolidated Investment Vehicles
Assets:
FG VIEs’ assets	309	269
Recurring | Level 3 | Fixed-maturity securities, available-for-sale
Assets:
Estimated Fair Value	980	963
Fixed-maturity securities, trading	0	0
Recurring | Level 3 | Fixed-maturity securities, available-for-sale | Obligations of state and political subdivisions
Assets:
Estimated Fair Value	6	6
Recurring | Level 3 | Fixed-maturity securities, available-for-sale | U.S. government and agencies
Assets:
Estimated Fair Value	0	0
Recurring | Level 3 | Fixed-maturity securities, available-for-sale | Corporate securities
Assets:
Estimated Fair Value	0	0
Recurring | Level 3 | Fixed-maturity securities, available-for-sale | RMBS
Assets:
Estimated Fair Value	153	154
Recurring | Level 3 | Fixed-maturity securities, available-for-sale | CMBS
Assets:
Estimated Fair Value	0	0
Recurring | Level 3 | Fixed-maturity securities, available-for-sale | Asset-backed securities
Assets:
Estimated Fair Value	821	803
Recurring | Level 3 | Fixed-maturity securities, available-for-sale | Non-U.S. government securities
Assets:
Estimated Fair Value	0	0
Recurring | Level 3 | Fund Investments | Equity securities and warrants | Consolidated Investment Vehicles
Assets:
FG VIEs’ assets	89	80
Recurring | Level 3 | Fund Investments | Structured products | Consolidated Investment Vehicles
Assets:
FG VIEs’ assets	$ 220	$ 189