Fair Value of Assets and Liabilities - Schedule of Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024
Assets
Fixed income securities, at fair value	$ 51,993	$ 52,747
Equity securities	4,465	4,463
Short-term investments	6,541	4,537
Other investments	901	824
Assets held for sale	3,786	3,092
Total assets at fair value	64,946	63,521
Counterparty and cash collateral netting	(1)	(19)
Total	65,958	64,617
Liabilities
Preferred stock measured at cost	150	150
U.S. government and agencies
Assets
Fixed income securities, at fair value	10,705	11,108
Municipal
Assets
Fixed income securities, at fair value	7,474	8,842
Foreign government
Assets
Fixed income securities, at fair value	1,334	1,364
ABS
Assets
Fixed income securities, at fair value	1,713	1,241
Quoted prices in active markets for identical assets (Level 1)
Assets
Total assets at fair value	15,540	16,956
Significant other observable inputs (Level 2)
Assets
Total assets at fair value	48,560	45,779
Significant unobservable inputs (Level 3)
Assets
Total assets at fair value	847	805
Reported to NAV
Assets
Investments reported at NAV	1,012	1,096
Recurring basis
Assets
Fixed income securities, at fair value	51,993	52,747
Equity securities	4,315	4,313
Short-term investments	6,541	4,537
Other investments	2	3
Other investments	(1)	(19)
Other assets	135	134
Assets held for sale	1,958	1,784
Total assets at fair value	64,944	63,518
Counterparty and cash collateral netting	(1)	(19)
Liabilities
Other liabilities	(1)	(1)
Other liabilities	7	1
Counterparty and cash collateral netting	7	1
Total recurring basis liabilities	(1)	(1)
Recurring basis | U.S. government and agencies
Assets
Fixed income securities, at fair value	10,705	11,108
Recurring basis | Municipal
Assets
Fixed income securities, at fair value	7,474	8,842
Recurring basis | Corporate - public
Assets
Fixed income securities, at fair value	21,440	21,233
Recurring basis | Corporate - privately placed
Assets
Fixed income securities, at fair value	9,327	8,959
Recurring basis | Foreign government
Assets
Fixed income securities, at fair value	1,334	1,364
Recurring basis | ABS
Assets
Fixed income securities, at fair value	1,713	1,241
Recurring basis | Quoted prices in active markets for identical assets (Level 1)
Assets
Fixed income securities, at fair value	10,697	11,099
Equity securities	3,586	3,600
Short-term investments	992	2,016
Other investments	0	0
Other assets	1	0
Assets held for sale	264	241
Total assets at fair value	15,540	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	10,697	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 | Significant other observable inputs (Level 2)
Assets
Fixed income securities, at fair value	41,011	41,400
Equity securities	313	306
Short-term investments	5,547	2,516
Other investments	2	21
Other assets	0	0
Assets held for sale	1,687	1,536
Total assets at fair value	48,560	45,779
Liabilities
Other liabilities	(7)	(1)
Total recurring basis liabilities	(7)	(1)
Recurring basis | Significant other observable inputs (Level 2) | U.S. government and agencies
Assets
Fixed income securities, at fair value	8	9
Recurring basis | Significant other observable inputs (Level 2) | Municipal
Assets
Fixed income securities, at fair value	7,472	8,840
Recurring basis | Significant other observable inputs (Level 2) | Corporate - public
Assets
Fixed income securities, at fair value	21,405	21,211
Recurring basis | Significant other observable inputs (Level 2) | Corporate - privately placed
Assets
Fixed income securities, at fair value	9,218	8,849
Recurring basis | Significant other observable inputs (Level 2) | Foreign government
Assets
Fixed income securities, at fair value	1,334	1,364
Recurring basis | Significant other observable inputs (Level 2) | ABS
Assets
Fixed income securities, at fair value	1,574	1,127
Recurring basis | Significant unobservable inputs (Level 3)
Assets
Fixed income securities, at fair value	285	248
Equity securities	416	407
Short-term investments	2	5
Other investments	1	1
Other assets	134	134
Assets held for sale	7	7
Total assets at fair value	845	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	2	2
Recurring basis | Significant unobservable inputs (Level 3) | Corporate - public
Assets
Fixed income securities, at fair value	35	22
Recurring basis | Significant unobservable inputs (Level 3) | Corporate - privately placed
Assets
Fixed income securities, at fair value	109	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	139	114
Non-recurring basis
Assets
Total assets at fair value	2	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	$ 2	$ 3