Fair Value of Assets and Liabilities - Schedule of Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Assets
Fixed income securities, at fair value	$ 50,777	$ 48,865
Equity securities	2,383	2,411
Short-term investments	4,318	5,144
Other investments	1,004	1,055
Total assets at fair value	57,452	56,409
Counterparty and cash collateral netting	$ (4)	$ (2)
% of total assets at fair value	0.00%	0.00%
Total	$ 58,598	$ 57,574
Liabilities
% 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, at fair value	$ 10,030	$ 8,619
Municipal
Assets
Fixed income securities, at fair value	5,392	6,006
Foreign government
Assets
Fixed income securities, at fair value	1,240	1,290
ABS
Assets
Fixed income securities, at fair value	1,846	1,745
Quoted prices in active markets for identical assets (Level 1)
Assets
Total assets at fair value	$ 13,744	$ 11,941
% of total assets at fair value	23.90%	21.20%
Liabilities
% of total liabilities at fair value	66.70%	50.00%
Significant other observable inputs (Level 2)
Assets
Total assets at fair value	$ 43,018	$ 43,779
% of total assets at fair value	74.90%	77.60%
Liabilities
% of total liabilities at fair value	133.30%	250.00%
Significant unobservable inputs (Level 3)
Assets
Total assets at fair value	$ 694	$ 691
% of total assets at fair value	1.20%	1.20%
Liabilities
% of total liabilities at fair value	0.00%	0.00%
Fair Value Net Asset Value
Assets
Investments reported at NAV	$ 1,146	$ 1,165
Recurring basis
Assets
Fixed income securities, at fair value	50,777	48,865
Equity securities	2,233	2,261
Short-term investments	4,318	5,144
Other investments	3	3
Other investments	(4)	(2)
Other assets	121	121
Total assets at fair value	57,452	56,394
Counterparty and cash collateral netting	(4)	(2)
Liabilities
Other liabilities	(3)	(4)
Other liabilities	3	8
Total recurring basis liabilities	(3)	(4)
Counterparty and cash collateral netting	3	8
Recurring basis | U.S. government and agencies
Assets
Fixed income securities, at fair value	10,030	8,619
Recurring basis | Municipal
Assets
Fixed income securities, at fair value	5,392	6,006
Recurring basis | Corporate - public
Assets
Fixed income securities, at fair value	23,870	23,298
Recurring basis | Corporate - privately placed
Assets
Fixed income securities, at fair value	8,399	7,907
Recurring basis | Foreign government
Assets
Fixed income securities, at fair value	1,240	1,290
Recurring basis | ABS
Assets
Fixed income securities, at fair value	1,846	1,745
Recurring basis | Quoted prices in active markets for identical assets (Level 1)
Assets
Fixed income securities, at fair value	10,016	8,606
Equity securities	1,607	1,656
Short-term investments	2,120	1,676
Other investments	0	0
Other assets	1	3
Total assets at fair value	13,744	11,941
Liabilities
Other liabilities	(2)	(2)
Total recurring basis liabilities	(2)	(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	10,016	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	40,618	40,106
Equity securities	218	203
Short-term investments	2,177	3,467
Other investments	5	3
Other assets	0	0
Total assets at fair value	43,018	43,779
Liabilities
Other liabilities	(4)	(10)
Total recurring basis liabilities	(4)	(10)
Recurring basis | Significant other observable inputs (Level 2) | U.S. government and agencies
Assets
Fixed income securities, at fair value	14	13
Recurring basis | Significant other observable inputs (Level 2) | Municipal
Assets
Fixed income securities, at fair value	5,384	5,995
Recurring basis | Significant other observable inputs (Level 2) | Corporate - public
Assets
Fixed income securities, at fair value	23,847	23,272
Recurring basis | Significant other observable inputs (Level 2) | Corporate - privately placed
Assets
Fixed income securities, at fair value	8,345	7,849
Recurring basis | Significant other observable inputs (Level 2) | Foreign government
Assets
Fixed income securities, at fair value	1,240	1,290
Recurring basis | Significant other observable inputs (Level 2) | ABS
Assets
Fixed income securities, at fair value	1,788	1,687
Recurring basis | Significant unobservable inputs (Level 3)
Assets
Fixed income securities, at fair value	143	153
Equity securities	408	402
Short-term investments	21	1
Other investments	2	2
Other assets	120	118
Total assets at fair value	694	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	8	11
Recurring basis | Significant unobservable inputs (Level 3) | Corporate - public
Assets
Fixed income securities, at fair value	23	26
Recurring basis | Significant unobservable inputs (Level 3) | Corporate - privately placed
Assets
Fixed income securities, at fair value	54	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	58	58
Non-recurring basis
Assets
Total assets at fair value	0	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	$ 0	$ 15