Fair Value of Assets and Liabilities - Assets and liabilities measured at fair value on a recurring and non-recurring basis (Details) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Assets
Fixed income securities, at fair value	$ 40,745	$ 42,136
Equity securities, at fair value	5,315	7,061
Short-term investments, at fair value	4,344	4,009
Other investments	2,532	2,656
Total assets at fair value	50,526	53,318
Counterparty and cash collateral netting	$ (27)	$ (22)
Assets as a percent of assets measured at fair value	(0.10%)	0.00%
Total	$ 52,016	$ 54,849
Liabilities
Liabilities as a percent of liabilities measured at fair value	(20.60%)	(87.50%)
Fair value
Assets
Assets as a percent of assets measured at fair value	100.00%	100.00%
Liabilities
Liabilities as a percent of liabilities measured at fair value	100.00%	100.00%
U.S. government and agencies
Assets
Fixed income securities, at fair value	$ 6,485	$ 6,273
Municipal
Assets
Fixed income securities, at fair value	5,698	6,393
Foreign government
Assets
Fixed income securities, at fair value	1,053	985
ABS
Assets
Fixed income securities, at fair value	2,173	1,155
Quoted prices in active markets for identical assets (Level 1)
Assets
Total assets at fair value	$ 12,168	$ 13,700
Assets as a percent of assets measured at fair value	24.10%	25.70%
Liabilities
Liabilities as a percent of liabilities measured at fair value	38.20%	37.50%
Significant other observable inputs (Level 2)
Assets
Total assets at fair value	$ 37,675	$ 39,043
Assets as a percent of assets measured at fair value	74.60%	73.20%
Liabilities
Liabilities as a percent of liabilities measured at fair value	82.40%	150.00%
Significant unobservable inputs (Level 3)
Assets
Total assets at fair value	$ 710	$ 597
Assets as a percent of assets measured at fair value	1.40%	1.10%
Liabilities
Liabilities as a percent of liabilities measured at fair value	0.00%	0.00%
Fair Value Net Asset Value
Assets
Investments reported at NAV	$ 1,490	$ 1,531
Recurring basis
Assets
Fixed income securities, at fair value	40,745	42,136
Equity securities, at fair value	5,315	7,061
Short-term investments, at fair value	4,344	4,009
Other investments	9	14
Other investments	(27)	(22)
Other assets	81	66
Total assets at fair value	50,494	53,286
Counterparty and cash collateral netting	(27)	(22)
Liabilities
Other liabilities	(34)	(8)
Other liabilities	7	7
Total recurring basis liabilities	(34)	(8)
Counterparty and cash collateral netting	7	7
Recurring basis | U.S. government and agencies
Assets
Fixed income securities, at fair value	6,485	6,273
Recurring basis | Municipal
Assets
Fixed income securities, at fair value	5,698	6,393
Recurring basis | Corporate - public
Assets
Fixed income securities, at fair value	15,508	16,589
Recurring basis | Corporate - privately placed
Assets
Fixed income securities, at fair value	9,828	10,741
Recurring basis | Foreign government
Assets
Fixed income securities, at fair value	1,053	985
Recurring basis | ABS
Assets
Fixed income securities, at fair value	2,173	1,155
Recurring basis | Quoted prices in active markets for identical assets (Level 1)
Assets
Fixed income securities, at fair value	6,459	6,247
Equity securities, at fair value	4,575	6,312
Short-term investments, at fair value	1,130	1,140
Other investments	0	0
Other assets	4	1
Total assets at fair value	12,168	13,700
Liabilities
Other liabilities	(13)	(3)
Total recurring basis liabilities	(13)	(3)
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | U.S. government and agencies
Assets
Fixed income securities, at fair value	6,459	6,247
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	34,071	35,745
Equity securities, at fair value	367	400
Short-term investments, at fair value	3,203	2,864
Other investments	34	34
Other assets	0	0
Total assets at fair value	37,675	39,043
Liabilities
Other liabilities	(28)	(12)
Total recurring basis liabilities	(28)	(12)
Recurring basis | Significant other observable inputs (Level 2) | U.S. government and agencies
Assets
Fixed income securities, at fair value	26	26
Recurring basis | Significant other observable inputs (Level 2) | Municipal
Assets
Fixed income securities, at fair value	5,681	6,375
Recurring basis | Significant other observable inputs (Level 2) | Corporate - public
Assets
Fixed income securities, at fair value	15,459	16,569
Recurring basis | Significant other observable inputs (Level 2) | Corporate - privately placed
Assets
Fixed income securities, at fair value	9,698	10,675
Recurring basis | Significant other observable inputs (Level 2) | Foreign government
Assets
Fixed income securities, at fair value	1,053	985
Recurring basis | Significant other observable inputs (Level 2) | ABS
Assets
Fixed income securities, at fair value	2,154	1,115
Recurring basis | Significant unobservable inputs (Level 3)
Assets
Fixed income securities, at fair value	215	144
Equity securities, at fair value	373	349
Short-term investments, at fair value	11	5
Other investments	2	2
Other assets	77	65
Total assets at fair value	678	565
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	17	18
Recurring basis | Significant unobservable inputs (Level 3) | Corporate - public
Assets
Fixed income securities, at fair value	49	20
Recurring basis | Significant unobservable inputs (Level 3) | Corporate - privately placed
Assets
Fixed income securities, at fair value	130	66
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	19	40
Non-recurring basis
Assets
Total assets at fair value	32	32
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	$ 32	$ 32