Fair Value of Assets and Liabilities - Assets and liabilities measured at fair value on a recurring and non-recurring basis (Details) - USD ($) $ in Millions	Sep. 30, 2022	Dec. 31, 2021
Assets
Total fixed income securities	$ 41,715	$ 42,136
Equity securities	4,723	7,061
Short-term investments	4,030	4,009
Other investments	1,798	2,656
Total assets at fair value	50,659	53,318
Counterparty and cash collateral netting	$ (59)	$ (22)
% of total assets at fair value	(0.10%)	0.00%
Total	$ 51,977	$ 54,849
Liabilities
% of total liabilities at fair value	14.60%	(87.50%)
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
Total fixed income securities	$ 8,444	$ 6,273
Municipal
Assets
Total fixed income securities	6,029	6,393
Foreign government
Assets
Total fixed income securities	890	985
ABS
Assets
Total fixed income securities	1,620	1,155
Quoted prices in active markets for identical assets (Level 1)
Assets
Total assets at fair value	$ 13,458	$ 13,700
% of total assets at fair value	26.60%	25.70%
Liabilities
% of total liabilities at fair value	18.70%	37.50%
Significant other observable inputs (Level 2)
Assets
Total assets at fair value	$ 36,592	$ 39,043
% of total assets at fair value	72.20%	73.20%
Liabilities
% of total liabilities at fair value	66.70%	150.00%
Significant unobservable inputs (Level 3)
Assets
Total assets at fair value	$ 668	$ 597
% of total assets at fair value	1.30%	1.10%
Liabilities
% of total liabilities at fair value	0.00%	0.00%
Fair Value Net Asset Value
Assets
Investments reported at NAV	$ 1,318	$ 1,531
Recurring basis
Assets
Total fixed income securities	41,715	42,136
Equity securities	4,723	7,061
Short-term investments	4,030	4,009
Other investments	50	14
Other investments	(59)	(22)
Other assets	118	66
Total assets at fair value	50,636	53,286
Counterparty and cash collateral netting	(59)	(22)
Liabilities
Other liabilities	(48)	(8)
Other liabilities	(7)	7
Total recurring basis liabilities	(48)	(8)
Counterparty and cash collateral netting	(7)	7
Recurring basis | U.S. government and agencies
Assets
Total fixed income securities	8,444	6,273
Recurring basis | Municipal
Assets
Total fixed income securities	6,029	6,393
Recurring basis | Corporate - public
Assets
Total fixed income securities	16,819	16,589
Recurring basis | Corporate - privately placed
Assets
Total fixed income securities	7,913	10,741
Recurring basis | Foreign government
Assets
Total fixed income securities	890	985
Recurring basis | ABS
Assets
Total fixed income securities	1,620	1,155
Recurring basis | Quoted prices in active markets for identical assets (Level 1)
Assets
Total fixed income securities	8,424	6,247
Equity securities	4,092	6,312
Short-term investments	928	1,140
Other investments	0	0
Other assets	14	1
Total assets at fair value	13,458	13,700
Liabilities
Other liabilities	(9)	(3)
Total recurring basis liabilities	(9)	(3)
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | U.S. government and agencies
Assets
Total fixed income securities	8,424	6,247
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | Municipal
Assets
Total fixed income securities	0	0
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | Corporate - public
Assets
Total fixed income securities	0	0
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | Corporate - privately placed
Assets
Total fixed income securities	0	0
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | Foreign government
Assets
Total fixed income securities	0	0
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | ABS
Assets
Total fixed income securities	0	0
Recurring basis | Significant other observable inputs (Level 2)
Assets
Total fixed income securities	33,085	35,745
Equity securities	304	400
Short-term investments	3,096	2,864
Other investments	107	34
Other assets	0	0
Total assets at fair value	36,592	39,043
Liabilities
Other liabilities	(32)	(12)
Total recurring basis liabilities	(32)	(12)
Recurring basis | Significant other observable inputs (Level 2) | U.S. government and agencies
Assets
Total fixed income securities	20	26
Recurring basis | Significant other observable inputs (Level 2) | Municipal
Assets
Total fixed income securities	6,010	6,375
Recurring basis | Significant other observable inputs (Level 2) | Corporate - public
Assets
Total fixed income securities	16,751	16,569
Recurring basis | Significant other observable inputs (Level 2) | Corporate - privately placed
Assets
Total fixed income securities	7,842	10,675
Recurring basis | Significant other observable inputs (Level 2) | Foreign government
Assets
Total fixed income securities	890	985
Recurring basis | Significant other observable inputs (Level 2) | ABS
Assets
Total fixed income securities	1,572	1,115
Recurring basis | Significant unobservable inputs (Level 3)
Assets
Total fixed income securities	206	144
Equity securities	327	349
Short-term investments	6	5
Other investments	2	2
Other assets	104	65
Total assets at fair value	645	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
Total fixed income securities	0	0
Recurring basis | Significant unobservable inputs (Level 3) | Municipal
Assets
Total fixed income securities	19	18
Recurring basis | Significant unobservable inputs (Level 3) | Corporate - public
Assets
Total fixed income securities	68	20
Recurring basis | Significant unobservable inputs (Level 3) | Corporate - privately placed
Assets
Total fixed income securities	71	66
Recurring basis | Significant unobservable inputs (Level 3) | Foreign government
Assets
Total fixed income securities	0	0
Recurring basis | Significant unobservable inputs (Level 3) | ABS
Assets
Total fixed income securities	48	40
Non-recurring basis
Assets
Total assets at fair value	23	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	$ 23	$ 32