Fair Value of Assets and Liabilities - Schedule of Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Assets
Fixed income securities	$ 52,747	$ 48,865
Equity securities	4,463	2,411
Short-term investments	4,537	5,144
Other investments	824	1,055
Assets held for sale	3,092	0
Counterparty and cash collateral netting	(19)	(2)
Total assets at fair value	$ 63,521	$ 56,409
% of total assets at fair value	0.00%	0.00%
Total	$ 64,617	$ 57,574
Liabilities
Other liabilities	$ (215)	$ (209)
% of total liabilities at fair value	(100.00%)	(200.00%)
Preferred stock measured at cost	$ 150	$ 150
Fair value
Assets
% of total assets at fair value	100.00%	100.00%
Liabilities
% of total liabilities at fair value	100.00%	100.00%
U.S. government and agencies
Assets
Fixed income securities	$ 11,108	$ 8,619
Municipal
Assets
Fixed income securities	8,842	6,006
Foreign government
Assets
Fixed income securities	1,364	1,290
ABS
Assets
Fixed income securities	1,241	1,745
Quoted prices in active markets for identical assets (Level 1)
Assets
Total assets at fair value	$ 16,956	$ 11,941
% of total assets at fair value	26.70%	21.20%
Liabilities
% of total liabilities at fair value	100.00%	50.00%
Significant other observable inputs (Level 2)
Assets
Total assets at fair value	$ 45,779	$ 43,779
% of total assets at fair value	72.00%	77.60%
Liabilities
% of total liabilities at fair value	100.00%	250.00%
Significant unobservable inputs (Level 3)
Assets
Total assets at fair value	$ 805	$ 691
% of total assets at fair value	1.30%	1.20%
Liabilities
% of total liabilities at fair value	0.00%	0.00%
Reported at NAV
Assets
Investments reported at NAV	$ 1,096	$ 1,165
Recurring Basis
Assets
Fixed income securities	52,747	48,865
Equity securities	4,313	2,261
Short-term investments	4,537	5,144
Other investments	3	3
Other investments	(19)	(2)
Other assets	134	121
Assets held for sale	1,784
Counterparty and cash collateral netting	(19)	(2)
Total assets at fair value	63,518	56,394
Liabilities
Other liabilities	(1)	(4)
Other liabilities	1	8
Counterparty and cash collateral netting	1	8
Total recurring basis liabilities	(1)	(4)
Recurring Basis | U.S. government and agencies
Assets
Fixed income securities	11,108	8,619
Recurring Basis | Municipal
Assets
Fixed income securities	8,842	6,006
Recurring Basis | Corporate - public
Assets
Fixed income securities	21,233	23,298
Recurring Basis | Corporate - privately placed
Assets
Fixed income securities	8,959	7,907
Recurring Basis | Foreign government
Assets
Fixed income securities	1,364	1,290
Recurring Basis | ABS
Assets
Fixed income securities	1,241	1,745
Recurring Basis | Quoted prices in active markets for identical assets (Level 1)
Assets
Fixed income securities	11,099	8,606
Equity securities	3,600	1,656
Short-term investments	2,016	1,676
Other investments	0	0
Other assets	0	3
Assets held for sale	241
Total assets at fair value	16,956	11,941
Liabilities
Other liabilities	(1)	(2)
Total recurring basis liabilities	(1)	(2)
Recurring Basis | Quoted prices in active markets for identical assets (Level 1) | U.S. government and agencies
Assets
Fixed income securities	11,099	8,606
Recurring Basis | Quoted prices in active markets for identical assets (Level 1) | Municipal
Assets
Fixed income securities	0	0
Recurring Basis | Quoted prices in active markets for identical assets (Level 1) | Corporate - public
Assets
Fixed income securities	0	0
Recurring Basis | Quoted prices in active markets for identical assets (Level 1) | Corporate - privately placed
Assets
Fixed income securities	0	0
Recurring Basis | Quoted prices in active markets for identical assets (Level 1) | Foreign government
Assets
Fixed income securities	0	0
Recurring Basis | Quoted prices in active markets for identical assets (Level 1) | ABS
Assets
Fixed income securities	0	0
Recurring Basis | Significant other observable inputs (Level 2)
Assets
Fixed income securities	41,400	40,106
Equity securities	306	203
Short-term investments	2,516	3,467
Other investments	21	3
Other assets	0	0
Assets held for sale	1,536
Total assets at fair value	45,779	43,779
Liabilities
Other liabilities	(1)	(10)
Total recurring basis liabilities	(1)	(10)
Recurring Basis | Significant other observable inputs (Level 2) | U.S. government and agencies
Assets
Fixed income securities	9	13
Recurring Basis | Significant other observable inputs (Level 2) | Municipal
Assets
Fixed income securities	8,840	5,995
Recurring Basis | Significant other observable inputs (Level 2) | Corporate - public
Assets
Fixed income securities	21,211	23,272
Recurring Basis | Significant other observable inputs (Level 2) | Corporate - privately placed
Assets
Fixed income securities	8,849	7,849
Recurring Basis | Significant other observable inputs (Level 2) | Foreign government
Assets
Fixed income securities	1,364	1,290
Recurring Basis | Significant other observable inputs (Level 2) | ABS
Assets
Fixed income securities	1,127	1,687
Recurring Basis | Significant unobservable inputs (Level 3)
Assets
Fixed income securities	248	153
Equity securities	407	402
Short-term investments	5	1
Other investments	1	2
Other assets	134	118
Assets held for sale	7
Total assets at fair value	802	676
Liabilities
Other liabilities	0	0
Total recurring basis liabilities	0	0
Recurring Basis | Significant unobservable inputs (Level 3) | U.S. government and agencies
Assets
Fixed income securities	0	0
Recurring Basis | Significant unobservable inputs (Level 3) | Municipal
Assets
Fixed income securities	2	11
Recurring Basis | Significant unobservable inputs (Level 3) | Corporate - public
Assets
Fixed income securities	22	26
Recurring Basis | Significant unobservable inputs (Level 3) | Corporate - privately placed
Assets
Fixed income securities	110	58
Recurring Basis | Significant unobservable inputs (Level 3) | Foreign government
Assets
Fixed income securities	0	0
Recurring Basis | Significant unobservable inputs (Level 3) | ABS
Assets
Fixed income securities	114	58
Non-recurring basis
Assets
Total assets at fair value	3	15
Non-recurring basis | Quoted prices in active markets for identical assets (Level 1)
Assets
Total assets at fair value	0	0
Non-recurring basis | Significant other observable inputs (Level 2)
Assets
Total assets at fair value	0	0
Non-recurring basis | Significant unobservable inputs (Level 3)
Assets
Total assets at fair value	$ 3	$ 15