Fair Value of Assets and Liabilities - Assets and liabilities measured at fair value on a recurring and non-recurring basis (Details) - USD ($) $ in Millions	Jun. 30, 2022	Dec. 31, 2021
Assets
Total fixed income securities	$ 41,282	$ 42,136
Equity securities	4,681	7,061
Short-term investments	4,384	4,009
Other investments	1,917	2,656
Total assets at fair value	50,545	53,318
Counterparty and cash collateral netting	$ (53)	$ (22)
% of total assets at fair value	(0.10%)	0.00%
Total	$ 51,940	$ 54,849
Liabilities
% of total liabilities at fair value	(10.00%)	(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	$ 8,745	$ 6,273
Municipal
Assets
Total fixed income securities	5,911	6,393
Foreign government
Assets
Total fixed income securities	972	985
ABS
Assets
Total fixed income securities	1,562	1,155
Quoted prices in active markets for identical assets (Level 1)
Assets
Total assets at fair value	$ 13,895	$ 13,700
% of total assets at fair value	27.50%	25.70%
Liabilities
% of total liabilities at fair value	65.00%	37.50%
Significant other observable inputs (Level 2)
Assets
Total assets at fair value	$ 35,995	$ 39,043
% of total assets at fair value	71.20%	73.20%
Liabilities
% of total liabilities at fair value	45.00%	150.00%
Significant unobservable inputs (Level 3)
Assets
Total assets at fair value	$ 708	$ 597
% of total assets at fair value	1.40%	1.10%
Liabilities
% of total liabilities at fair value	0.00%	0.00%
Fair Value Net Asset Value
Assets
Investments reported at NAV	$ 1,395	$ 1,531
Recurring basis
Assets
Total fixed income securities	41,282	42,136
Equity securities	4,681	7,061
Short-term investments	4,384	4,009
Other investments	55	14
Other investments	(53)	(22)
Other assets	111	66
Total assets at fair value	50,513	53,286
Counterparty and cash collateral netting	(53)	(22)
Liabilities
Other liabilities	(60)	(8)
Other liabilities	6	7
Total recurring basis liabilities	(60)	(8)
Counterparty and cash collateral netting	6	7
Recurring basis | U.S. government and agencies
Assets
Total fixed income securities	8,745	6,273
Recurring basis | Municipal
Assets
Total fixed income securities	5,911	6,393
Recurring basis | Corporate - public
Assets
Total fixed income securities	15,891	16,589
Recurring basis | Corporate - privately placed
Assets
Total fixed income securities	8,201	10,741
Recurring basis | Foreign government
Assets
Total fixed income securities	972	985
Recurring basis | ABS
Assets
Total fixed income securities	1,562	1,155
Recurring basis | Quoted prices in active markets for identical assets (Level 1)
Assets
Total fixed income securities	8,721	6,247
Equity securities	3,963	6,312
Short-term investments	1,208	1,140
Other investments	0	0
Other assets	3	1
Total assets at fair value	13,895	13,700
Liabilities
Other liabilities	(39)	(3)
Total recurring basis liabilities	(39)	(3)
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | U.S. government and agencies
Assets
Total fixed income securities	8,721	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	32,375	35,745
Equity securities	346	400
Short-term investments	3,168	2,864
Other investments	106	34
Other assets	0	0
Total assets at fair value	35,995	39,043
Liabilities
Other liabilities	(27)	(12)
Total recurring basis liabilities	(27)	(12)
Recurring basis | Significant other observable inputs (Level 2) | U.S. government and agencies
Assets
Total fixed income securities	24	26
Recurring basis | Significant other observable inputs (Level 2) | Municipal
Assets
Total fixed income securities	5,893	6,375
Recurring basis | Significant other observable inputs (Level 2) | Corporate - public
Assets
Total fixed income securities	15,814	16,569
Recurring basis | Significant other observable inputs (Level 2) | Corporate - privately placed
Assets
Total fixed income securities	8,128	10,675
Recurring basis | Significant other observable inputs (Level 2) | Foreign government
Assets
Total fixed income securities	972	985
Recurring basis | Significant other observable inputs (Level 2) | ABS
Assets
Total fixed income securities	1,544	1,115
Recurring basis | Significant unobservable inputs (Level 3)
Assets
Total fixed income securities	186	144
Equity securities	372	349
Short-term investments	8	5
Other investments	2	2
Other assets	108	65
Total assets at fair value	676	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	18	18
Recurring basis | Significant unobservable inputs (Level 3) | Corporate - public
Assets
Total fixed income securities	77	20
Recurring basis | Significant unobservable inputs (Level 3) | Corporate - privately placed
Assets
Total fixed income securities	73	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	18	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