Fair Value Measurement - Financial Instruments Carried at Fair Value (Details) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024
Assets:
Estimated Fair Value	$ 6,415	$ 6,369
Fixed-maturity securities, trading	137	147
Short-term investments, at fair value	1,158	1,221
Other invested assets	960	926
Other assets	134	131
Liabilities:
FG VIEs' liabilities	163	164
Other liabilities. fair value disclosure	34	34
Obligations of state and political subdivisions
Assets:
Estimated Fair Value	1,924	1,940
U.S. government and agencies
Assets:
Estimated Fair Value	72	67
Corporate securities
Assets:
Estimated Fair Value	2,477	2,382
RMBS
Assets:
Estimated Fair Value	569	567
CMBS
Assets:
Estimated Fair Value	182	186
Non-U.S. government securities
Assets:
Estimated Fair Value	71	69
Recurring
Assets:
Estimated Fair Value	6,415	6,369
Fixed-maturity securities, trading	137	147
Short-term investments, at fair value	1,158	1,221
Other invested assets	3	4
Other assets	134	131
Total assets carried at fair value	8,110	8,118
Liabilities:
Other liabilities. fair value disclosure	34	34
Total liabilities carried at fair value	197	198
Recurring | Financial Guaranty Variable Interest Entities
Liabilities:
FG VIEs' liabilities	163	164
Recurring | Financial Guaranty Variable Interest Entities
Assets:
FG VIEs’ assets	145	147
Recurring | Obligations of state and political subdivisions
Assets:
Estimated Fair Value	1,924	1,940
Recurring | U.S. government and agencies
Assets:
Estimated Fair Value	72	67
Recurring | Corporate securities
Assets:
Estimated Fair Value	2,477	2,382
Recurring | RMBS
Assets:
Estimated Fair Value	569	567
Recurring | CMBS
Assets:
Estimated Fair Value	182	186
Recurring | Asset-backed securities
Assets:
Estimated Fair Value	1,120	1,158
Recurring | Non-U.S. government securities
Assets:
Estimated Fair Value	71	69
Recurring | Assets of CIVs, Equity Securities | Consolidated Investment Vehicles
Assets:
FG VIEs’ assets	118	99
Recurring | Level 1
Assets:
Estimated Fair Value	0	0
Fixed-maturity securities, trading	0	0
Short-term investments, at fair value	1,154	1,218
Other invested assets	0	0
Other assets	68	65
Total assets carried at fair value	1,222	1,283
Liabilities:
Other liabilities. fair value disclosure	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 | Obligations of state and political subdivisions
Assets:
Estimated Fair Value	0	0
Recurring | Level 1 | U.S. government and agencies
Assets:
Estimated Fair Value	0	0
Recurring | Level 1 | Corporate securities
Assets:
Estimated Fair Value	0	0
Recurring | Level 1 | RMBS
Assets:
Estimated Fair Value	0	0
Recurring | Level 1 | CMBS
Assets:
Estimated Fair Value	0	0
Recurring | Level 1 | Asset-backed securities
Assets:
Estimated Fair Value	0	0
Recurring | Level 1 | Non-U.S. government securities
Assets:
Estimated Fair Value	0	0
Recurring | Level 1 | Assets of CIVs, Equity Securities | Consolidated Investment Vehicles
Assets:
FG VIEs’ assets	0	0
Recurring | Level 2
Assets:
Estimated Fair Value	5,273	5,183
Fixed-maturity securities, trading	132	142
Short-term investments, at fair value	4	3
Other invested assets	0	0
Other assets	58	59
Total assets carried at fair value	5,467	5,387
Liabilities:
Other liabilities. fair value disclosure	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 | Obligations of state and political subdivisions
Assets:
Estimated Fair Value	1,914	1,930
Recurring | Level 2 | U.S. government and agencies
Assets:
Estimated Fair Value	72	67
Recurring | Level 2 | Corporate securities
Assets:
Estimated Fair Value	2,472	2,382
Recurring | Level 2 | RMBS
Assets:
Estimated Fair Value	425	422
Recurring | Level 2 | CMBS
Assets:
Estimated Fair Value	182	186
Recurring | Level 2 | Asset-backed securities
Assets:
Estimated Fair Value	137	127
Recurring | Level 2 | Non-U.S. government securities
Assets:
Estimated Fair Value	71	69
Recurring | Level 2 | Assets of CIVs, Equity Securities | Consolidated Investment Vehicles
Assets:
FG VIEs’ assets	0	0
Recurring | Level 3
Assets:
Estimated Fair Value	1,142	1,186
Fixed-maturity securities, trading	5	5
Short-term investments, at fair value	0	0
Other invested assets	3	4
Other assets	8	7
Total assets carried at fair value	1,421	1,448
Liabilities:
Other liabilities. fair value disclosure	34	34
Total liabilities carried at fair value	197	198
Recurring | Level 3 | Financial Guaranty Variable Interest Entities
Liabilities:
FG VIEs' liabilities	163	164
Recurring | Level 3 | Financial Guaranty Variable Interest Entities
Assets:
FG VIEs’ assets	145	147
Recurring | Level 3 | Obligations of state and political subdivisions
Assets:
Estimated Fair Value	10	10
Recurring | Level 3 | U.S. government and agencies
Assets:
Estimated Fair Value	0	0
Recurring | Level 3 | Corporate securities
Assets:
Estimated Fair Value	5	0
Recurring | Level 3 | RMBS
Assets:
Estimated Fair Value	144	145
Recurring | Level 3 | CMBS
Assets:
Estimated Fair Value	0	0
Recurring | Level 3 | Asset-backed securities
Assets:
Estimated Fair Value	983	1,031
Recurring | Level 3 | Non-U.S. government securities
Assets:
Estimated Fair Value	0	0
Recurring | Level 3 | Assets of CIVs, Equity Securities | Consolidated Investment Vehicles
Assets:
FG VIEs’ assets	$ 118	$ 99