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, 2021	Dec. 31, 2020
Assets
Fixed income securities, at fair value	$ 39,989	$ 42,565
Equity securities, at fair value	3,807	3,168
Short-term investments, at fair value	6,428	6,807
Other investments	3,286	1,691
Assets held for sale	36,803	42,131
Total assets at fair value	82,593	82,413
Counterparty and cash collateral netting	$ (22)	$ (15)
Assets as a percent of assets measured at fair value	0.00%	0.00%
Total	$ 84,697	$ 84,237
Liabilities
Liabilities held for sale	$ (32,421)	$ (33,325)
Liabilities as a percent of liabilities measured at fair value	0.50%	(4.20%)
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	$ 3,042	$ 2,107
Municipal
Assets
Fixed income securities, at fair value	7,417	7,578
Foreign government
Assets
Fixed income securities, at fair value	1,059	958
ABS
Assets
Fixed income securities, at fair value	1,070	846
MBS
Assets
Fixed income securities, at fair value	34	59
Quoted prices in active markets for identical assets (Level 1)
Assets
Total assets at fair value	$ 16,016	$ 17,567
Assets as a percent of assets measured at fair value	19.40%	21.30%
Liabilities
Liabilities as a percent of liabilities measured at fair value	0.70%	0.00%
Significant other observable inputs (Level 2)
Assets
Total assets at fair value	$ 65,771	$ 64,029
Assets as a percent of assets measured at fair value	79.60%	77.70%
Liabilities
Liabilities as a percent of liabilities measured at fair value	18.20%	23.80%
Significant unobservable inputs (Level 3)
Assets
Total assets at fair value	$ 828	$ 832
Assets as a percent of assets measured at fair value	1.00%	1.00%
Liabilities
Liabilities as a percent of liabilities measured at fair value	80.60%	80.40%
Significant unobservable inputs (Level 3) | Fair value
Assets
Assets held for sale	$ 3,050	$ 4,440
Liabilities
Liabilities held for sale	(8,248)	(9,424)
Fair Value Net Asset Value
Assets
Assets held for sale	549	762
Investments reported at NAV	1,555	1,062
Recurring basis
Assets
Fixed income securities, at fair value	39,989	42,565
Equity securities, at fair value	3,807	3,168
Short-term investments, at fair value	6,428	6,807
Other investments	22	20
Other investments	(19)	(9)
Other assets	4	1
Assets held for sale	32,252	29,852
Assets held for sale	(3)	(6)
Total assets at fair value	82,502	82,413
Counterparty and cash collateral netting	(22)	(15)
Liabilities
Other liabilities	(38)	(16)
Other liabilities	1	18
Liabilities held for sale	(556)	(626)
Liabilities held for sale counterparty and cash collateral netting	(4)	9
Total recurring basis liabilities	(594)	(642)
Counterparty and cash collateral netting	(3)	27
Recurring basis | U.S. government and agencies
Assets
Fixed income securities, at fair value	3,042	2,106
Recurring basis | Municipal
Assets
Fixed income securities, at fair value	7,417	7,579
Recurring basis | Corporate - public
Assets
Fixed income securities, at fair value	17,347	21,952
Recurring basis | Corporate - privately placed
Assets
Fixed income securities, at fair value	10,020	9,065
Recurring basis | Foreign government
Assets
Fixed income securities, at fair value	1,059	958
Recurring basis | ABS
Assets
Fixed income securities, at fair value	1,070	846
Recurring basis | MBS
Assets
Fixed income securities, at fair value	34	59
Recurring basis | Quoted prices in active markets for identical assets (Level 1)
Assets
Fixed income securities, at fair value	3,015	2,061
Equity securities, at fair value	3,025	2,468
Short-term investments, at fair value	3,681	6,549
Other investments	0	0
Other assets	4	1
Assets held for sale	6,291	6,488
Total assets at fair value	16,016	17,567
Liabilities
Other liabilities	(4)	0
Liabilities held for sale	0	0
Total recurring basis liabilities	(4)	0
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | U.S. government and agencies
Assets
Fixed income securities, at fair value	3,015	2,061
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 | Quoted prices in active markets for identical assets (Level 1) | MBS
Assets
Fixed income securities, at fair value	0	0
Recurring basis | Significant other observable inputs (Level 2)
Assets
Fixed income securities, at fair value	36,775	40,278
Equity securities, at fair value	406	396
Short-term investments, at fair value	2,733	223
Other investments	39	29
Other assets	0	0
Assets held for sale	25,818	23,103
Total assets at fair value	65,771	64,029
Liabilities
Other liabilities	(35)	(34)
Liabilities held for sale	(73)	(119)
Total recurring basis liabilities	(108)	(153)
Recurring basis | Significant other observable inputs (Level 2) | U.S. government and agencies
Assets
Fixed income securities, at fair value	27	45
Recurring basis | Significant other observable inputs (Level 2) | Municipal
Assets
Fixed income securities, at fair value	7,400	7,562
Recurring basis | Significant other observable inputs (Level 2) | Corporate - public
Assets
Fixed income securities, at fair value	17,328	21,885
Recurring basis | Significant other observable inputs (Level 2) | Corporate - privately placed
Assets
Fixed income securities, at fair value	9,866	9,002
Recurring basis | Significant other observable inputs (Level 2) | Foreign government
Assets
Fixed income securities, at fair value	1,059	958
Recurring basis | Significant other observable inputs (Level 2) | ABS
Assets
Fixed income securities, at fair value	1,069	794
Recurring basis | Significant other observable inputs (Level 2) | MBS
Assets
Fixed income securities, at fair value	26	32
Recurring basis | Significant unobservable inputs (Level 3)
Assets
Fixed income securities, at fair value	199	226
Equity securities, at fair value	376	304
Short-term investments, at fair value	14	35
Other investments	2	0
Other assets	0	0
Assets held for sale	146	267
Total assets at fair value	737	832
Liabilities
Other liabilities	0	0
Liabilities held for sale	(479)	(516)
Total recurring basis liabilities	(479)	(516)
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	17
Recurring basis | Significant unobservable inputs (Level 3) | Corporate - public
Assets
Fixed income securities, at fair value	19	67
Recurring basis | Significant unobservable inputs (Level 3) | Corporate - privately placed
Assets
Fixed income securities, at fair value	154	63
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	1	52
Recurring basis | Significant unobservable inputs (Level 3) | MBS
Assets
Fixed income securities, at fair value	8	$ 27
Non-recurring basis
Assets
Total assets at fair value	91
Non-recurring basis | Quoted prices in active markets for identical assets (Level 1)
Assets
Total assets at fair value	0
Non-recurring basis | Significant other observable inputs (Level 2)
Assets
Total assets at fair value	0
Non-recurring basis | Significant unobservable inputs (Level 3)
Assets
Total assets at fair value	$ 91