Fair Value of Assets and Liabilities - Schedule of Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Assets
Fixed income securities	$ 46,771	$ 42,485
Equity securities	2,419	4,567
Short-term investments	3,368	4,173
Other investments	1,608	1,728
Total assets at fair value	52,679	51,358
Counterparty and cash collateral netting	$ (16)	$ (22)
% of total assets at fair value	0.00%	0.00%
Total	$ 53,873	$ 52,604
Liabilities
% of total liabilities at fair value	(44.40%)	(420.00%)
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	$ 8,245	$ 7,898
Municipal
Assets
Fixed income securities	6,584	6,210
Foreign government
Assets
Fixed income securities	1,135	957
ABS
Assets
Fixed income securities	1,101	1,157
Quoted prices in active markets for identical assets (Level 1)
Assets
Total assets at fair value	$ 11,604	$ 12,325
% of total assets at fair value	22.00%	24.00%
Liabilities
% of total liabilities at fair value	33.30%	20.00%
Significant other observable inputs (Level 2)
Assets
Total assets at fair value	$ 40,444	$ 38,414
% of total assets at fair value	76.80%	74.80%
Liabilities
% of total liabilities at fair value	111.10%	500.00%
Significant unobservable inputs (Level 3)
Assets
Total assets at fair value	$ 647	$ 641
% 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	$ 1,194	$ 1,246
Recurring basis
Assets
Fixed income securities	46,771	42,485
Equity securities	2,419	4,567
Short-term investments	3,368	4,173
Other investments	6	4
Other investments	(16)	(22)
Other assets	115	106
Total assets at fair value	52,679	51,335
Counterparty and cash collateral netting	(16)	(22)
Liabilities
Other liabilities	(9)	(5)
Other liabilities	4	21
Total recurring basis liabilities	(9)	(5)
Counterparty and cash collateral netting	4	21
Recurring basis | U.S. government and agencies
Assets
Fixed income securities	8,245	7,898
Recurring basis | Municipal
Assets
Fixed income securities	6,584	6,210
Recurring basis | Corporate - public
Assets
Fixed income securities	22,059	18,616
Recurring basis | Corporate - privately placed
Assets
Fixed income securities	7,647	7,647
Recurring basis | Foreign government
Assets
Fixed income securities	1,135	957
Recurring basis | ABS
Assets
Fixed income securities	1,101	1,157
Recurring basis | Quoted prices in active markets for identical assets (Level 1)
Assets
Fixed income securities	8,232	7,878
Equity securities	1,892	3,936
Short-term investments	1,478	508
Other investments	0	0
Other assets	2	3
Total assets at fair value	11,604	12,325
Liabilities
Other liabilities	(3)	(1)
Total recurring basis liabilities	(3)	(1)
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | U.S. government and agencies
Assets
Fixed income securities	8,232	7,878
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	38,406	34,434
Equity securities	144	298
Short-term investments	1,874	3,659
Other investments	20	23
Other assets	0	0
Total assets at fair value	40,444	38,414
Liabilities
Other liabilities	(10)	(25)
Total recurring basis liabilities	(10)	(25)
Recurring basis | Significant other observable inputs (Level 2) | U.S. government and agencies
Assets
Fixed income securities	13	20
Recurring basis | Significant other observable inputs (Level 2) | Municipal
Assets
Fixed income securities	6,573	6,189
Recurring basis | Significant other observable inputs (Level 2) | Corporate - public
Assets
Fixed income securities	22,034	18,547
Recurring basis | Significant other observable inputs (Level 2) | Corporate - privately placed
Assets
Fixed income securities	7,588	7,592
Recurring basis | Significant other observable inputs (Level 2) | Foreign government
Assets
Fixed income securities	1,135	957
Recurring basis | Significant other observable inputs (Level 2) | ABS
Assets
Fixed income securities	1,063	1,129
Recurring basis | Significant unobservable inputs (Level 3)
Assets
Fixed income securities	133	173
Equity securities	383	333
Short-term investments	16	6
Other investments	2	3
Other assets	113	103
Total assets at fair value	647	618
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	11	21
Recurring basis | Significant unobservable inputs (Level 3) | Corporate - public
Assets
Fixed income securities	25	69
Recurring basis | Significant unobservable inputs (Level 3) | Corporate - privately placed
Assets
Fixed income securities	59	55
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	38	28
Non-recurring basis
Assets
Total assets at fair value	0	23
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	$ 23