Fair Value of Assets and Liabilities - Schedule of Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Assets
Fixed income securities, at fair value	$ 53,961	$ 48,865
Equity securities	2,091	2,411
Short-term investments	6,994	5,144
Other investments	866	1,055
Assets held for sale	3,163	0
Total assets at fair value	64,742	56,409
Counterparty and cash collateral netting	$ (1)	$ (2)
% of total assets at fair value	0.00%	0.00%
Total	$ 65,866	$ 57,574
Liabilities
% of total liabilities at fair value	(177.80%)	(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, at fair value	$ 9,246	$ 8,619
Municipal
Assets
Fixed income securities, at fair value	8,258	6,006
Foreign government
Assets
Fixed income securities, at fair value	1,477	1,290
ABS
Assets
Fixed income securities, at fair value	1,184	1,745
Quoted prices in active markets for identical assets (Level 1)
Assets
Total assets at fair value	$ 13,484	$ 11,941
% of total assets at fair value	20.80%	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	$ 50,535	$ 43,779
% of total assets at fair value	78.10%	77.60%
Liabilities
% of total liabilities at fair value	177.80%	250.00%
Significant unobservable inputs (Level 3)
Assets
Total assets at fair value	$ 724	$ 691
% of total assets at fair value	1.10%	1.20%
Liabilities
% of total liabilities at fair value	0.00%	0.00%
Fair Value Net Asset Value
Assets
Investments reported at NAV	$ 1,124	$ 1,165
Recurring basis
Assets
Fixed income securities, at fair value	53,961	48,865
Equity securities	1,941	2,261
Short-term investments	6,994	5,144
Other investments	2	3
Other investments	(1)	(2)
Other assets	125	121
Assets held for sale	1,718
Total assets at fair value	64,741	56,394
Counterparty and cash collateral netting	(1)	(2)
Liabilities
Other liabilities	(9)	(4)
Other liabilities	16	8
Total recurring basis liabilities	(9)	(4)
Counterparty and cash collateral netting	16	8
Recurring basis | U.S. government and agencies
Assets
Fixed income securities, at fair value	9,246	8,619
Recurring basis | Municipal
Assets
Fixed income securities, at fair value	8,258	6,006
Recurring basis | Corporate - public
Assets
Fixed income securities, at fair value	24,320	23,298
Recurring basis | Corporate - privately placed
Assets
Fixed income securities, at fair value	9,476	7,907
Recurring basis | Foreign government
Assets
Fixed income securities, at fair value	1,477	1,290
Recurring basis | ABS
Assets
Fixed income securities, at fair value	1,184	1,745
Recurring basis | Quoted prices in active markets for identical assets (Level 1)
Assets
Fixed income securities, at fair value	9,239	8,606
Equity securities	1,295	1,656
Short-term investments	2,771	1,676
Other investments	0	0
Other assets	2	3
Assets held for sale	177
Total assets at fair value	13,484	11,941
Liabilities
Other liabilities	(9)	(2)
Total recurring basis liabilities	(9)	(2)
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | U.S. government and agencies
Assets
Fixed income securities, at fair value	9,239	8,606
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	44,541	40,106
Equity securities	238	203
Short-term investments	4,221	3,467
Other investments	1	3
Other assets	0	0
Assets held for sale	1,534
Total assets at fair value	50,535	43,779
Liabilities
Other liabilities	(16)	(10)
Total recurring basis liabilities	(16)	(10)
Recurring basis | Significant other observable inputs (Level 2) | U.S. government and agencies
Assets
Fixed income securities, at fair value	7	13
Recurring basis | Significant other observable inputs (Level 2) | Municipal
Assets
Fixed income securities, at fair value	8,253	5,995
Recurring basis | Significant other observable inputs (Level 2) | Corporate - public
Assets
Fixed income securities, at fair value	24,292	23,272
Recurring basis | Significant other observable inputs (Level 2) | Corporate - privately placed
Assets
Fixed income securities, at fair value	9,425	7,849
Recurring basis | Significant other observable inputs (Level 2) | Foreign government
Assets
Fixed income securities, at fair value	1,477	1,290
Recurring basis | Significant other observable inputs (Level 2) | ABS
Assets
Fixed income securities, at fair value	1,087	1,687
Recurring basis | Significant unobservable inputs (Level 3)
Assets
Fixed income securities, at fair value	181	153
Equity securities	408	402
Short-term investments	2	1
Other investments	2	2
Other assets	123	118
Assets held for sale	7
Total assets at fair value	723	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, at fair value	0	0
Recurring basis | Significant unobservable inputs (Level 3) | Municipal
Assets
Fixed income securities, at fair value	5	11
Recurring basis | Significant unobservable inputs (Level 3) | Corporate - public
Assets
Fixed income securities, at fair value	28	26
Recurring basis | Significant unobservable inputs (Level 3) | Corporate - privately placed
Assets
Fixed income securities, at fair value	51	58
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	97	58
Non-recurring basis
Assets
Total assets at fair value	1	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	$ 1	$ 15