Fair Value of Assets and Liabilities - Schedule of Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024
Assets
Fixed income securities, at fair value	$ 57,186	$ 52,747
Equity securities	5,338	4,463
Short-term investments	8,743	4,537
Other investments	1,017	824
Assets held for sale	0	3,092
Total assets at fair value	71,430	63,521
Counterparty and cash collateral netting	(2)	(19)
Total	72,268	64,617
Liabilities
Preferred stock measured at cost	91	150
U.S. government and agencies
Assets
Fixed income securities, at fair value	13,525	11,108
Municipal
Assets
Fixed income securities, at fair value	6,226	8,842
Foreign government
Assets
Fixed income securities, at fair value	1,435	1,364
ABS
Assets
Fixed income securities, at fair value	986	1,145
MBS
Assets
Fixed income securities, at fair value	2,134	96
Quoted prices in active markets for identical assets (Level 1)
Assets
Total assets at fair value	21,630	16,956
Significant other observable inputs (Level 2)
Assets
Total assets at fair value	48,936	45,779
Significant unobservable inputs (Level 3)
Assets
Total assets at fair value	866	805
Reported to NAV
Assets
Investments reported at NAV	838	1,096
Recurring basis
Assets
Fixed income securities, at fair value	57,186	52,747
Equity securities	5,247	4,313
Short-term investments	8,743	4,537
Other investments	13	3
Other investments	(2)	(19)
Other assets	141	134
Assets held for sale		1,784
Total assets at fair value	71,330	63,518
Counterparty and cash collateral netting	(2)	(19)
Liabilities
Other liabilities	(2)	(1)
Other liabilities	30	1
Counterparty and cash collateral netting	30	1
Total recurring basis liabilities	(2)	(1)
Recurring basis | U.S. government and agencies
Assets
Fixed income securities, at fair value	13,525	11,108
Recurring basis | Municipal
Assets
Fixed income securities, at fair value	6,226	8,842
Recurring basis | Corporate - public
Assets
Fixed income securities, at fair value	21,887	21,233
Recurring basis | Corporate - privately placed
Assets
Fixed income securities, at fair value	10,993	8,959
Recurring basis | Foreign government
Assets
Fixed income securities, at fair value	1,435	1,364
Recurring basis | ABS
Assets
Fixed income securities, at fair value	986	1,145
Recurring basis | MBS
Assets
Fixed income securities, at fair value	2,134	96
Recurring basis | Quoted prices in active markets for identical assets (Level 1)
Assets
Fixed income securities, at fair value	13,516	11,099
Equity securities	4,570	3,600
Short-term investments	3,543	2,016
Other investments	0	0
Other assets	1	0
Assets held for sale		241
Total assets at fair value	21,630	16,956
Liabilities
Other liabilities	0	(1)
Total recurring basis liabilities	0	(1)
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | U.S. government and agencies
Assets
Fixed income securities, at fair value	13,516	11,099
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | Municipal
Assets
Fixed income securities, at fair value	0	0
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | Corporate - public
Assets
Fixed income securities, at fair value	0	0
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | Corporate - privately placed
Assets
Fixed income securities, at fair value	0	0
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | Foreign government
Assets
Fixed income securities, at fair value	0	0
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | ABS
Assets
Fixed income securities, at fair value	0	0
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | MBS
Assets
Fixed income securities, at fair value	0	0
Recurring basis | Significant other observable inputs (Level 2)
Assets
Fixed income securities, at fair value	43,364	41,400
Equity securities	367	306
Short-term investments	5,191	2,516
Other investments	14	21
Other assets	0	0
Assets held for sale		1,536
Total assets at fair value	48,936	45,779
Liabilities
Other liabilities	(31)	(1)
Total recurring basis liabilities	(31)	(1)
Recurring basis | Significant other observable inputs (Level 2) | U.S. government and agencies
Assets
Fixed income securities, at fair value	9	9
Recurring basis | Significant other observable inputs (Level 2) | Municipal
Assets
Fixed income securities, at fair value	6,223	8,840
Recurring basis | Significant other observable inputs (Level 2) | Corporate - public
Assets
Fixed income securities, at fair value	21,853	21,211
Recurring basis | Significant other observable inputs (Level 2) | Corporate - privately placed
Assets
Fixed income securities, at fair value	10,837	8,849
Recurring basis | Significant other observable inputs (Level 2) | Foreign government
Assets
Fixed income securities, at fair value	1,435	1,364
Recurring basis | Significant other observable inputs (Level 2) | ABS
Assets
Fixed income securities, at fair value	962	1,119
Recurring basis | Significant other observable inputs (Level 2) | MBS
Assets
Fixed income securities, at fair value	2,045	8
Recurring basis | Significant unobservable inputs (Level 3)
Assets
Fixed income securities, at fair value	306	248
Equity securities	310	407
Short-term investments	9	5
Other investments	1	1
Other assets	140	134
Assets held for sale		7
Total assets at fair value	766	802
Liabilities
Other liabilities	(1)	0
Total recurring basis liabilities	(1)	0
Recurring basis | Significant unobservable inputs (Level 3) | U.S. government and agencies
Assets
Fixed income securities, at fair value	0	0
Recurring basis | Significant unobservable inputs (Level 3) | Municipal
Assets
Fixed income securities, at fair value	3	2
Recurring basis | Significant unobservable inputs (Level 3) | Corporate - public
Assets
Fixed income securities, at fair value	34	22
Recurring basis | Significant unobservable inputs (Level 3) | Corporate - privately placed
Assets
Fixed income securities, at fair value	156	110
Recurring basis | Significant unobservable inputs (Level 3) | Foreign government
Assets
Fixed income securities, at fair value	0	0
Recurring basis | Significant unobservable inputs (Level 3) | ABS
Assets
Fixed income securities, at fair value	24	26
Recurring basis | Significant unobservable inputs (Level 3) | MBS
Assets
Fixed income securities, at fair value	89	88
Non-recurring basis
Assets
Total assets at fair value	100	3
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	$ 100	$ 3