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, 2021	Dec. 31, 2020
Assets
Fixed income securities, at fair value	$ 40,594	$ 42,565
Equity securities, at fair value	3,154	3,168
Short-term investments, at fair value	6,017	6,807
Other investments	3,042	1,691
Assets held for sale	36,829	42,131
Assets held for sale	(7)	(6)
Counterparty and cash collateral netting	$ (24)	$ (15)
Assets as a percent of assets measured at fair value	0.00%	0.00%
Total	$ 83,578	$ 84,237
Liabilities
Liabilities held for sale	(33,383)	(33,325)
Counterparty and cash collateral netting	17	27
Liabilities held for sale counterparty and cash collateral netting	$ 4	$ 9
Liabilities as a percent of liabilities measured at fair value	(3.00%)	(4.20%)
U.S. government and agencies
Assets
Fixed income securities, at fair value	$ 3,110	$ 2,107
Municipal
Assets
Fixed income securities, at fair value	7,500	7,578
Foreign government
Assets
Fixed income securities, at fair value	985	958
ABS
Assets
Fixed income securities, at fair value	943	846
MBS
Assets
Fixed income securities, at fair value	53	59
Quoted prices in active markets for identical assets (Level 1)
Assets
Total assets at fair value	$ 15,303	$ 17,567
Assets as a percent of assets measured at fair value	18.70%	21.30%
Liabilities
Total recurring basis liabilities	$ (1)	$ 0
Liabilities as a percent of liabilities measured at fair value	0.20%	0.00%
Significant other observable inputs (Level 2)
Assets
Total assets at fair value	$ 65,523	$ 64,029
Assets as a percent of assets measured at fair value	80.30%	77.70%
Liabilities
Total recurring basis liabilities	$ (118)	$ (153)
Liabilities as a percent of liabilities measured at fair value	20.90%	23.80%
Significant unobservable inputs (Level 3)
Assets
Total assets at fair value	$ 851	$ 832
Assets as a percent of assets measured at fair value	1.00%	1.00%
Liabilities
Total recurring basis liabilities	$ (463)	$ (516)
Liabilities as a percent of liabilities measured at fair value	81.90%	80.40%
Fair Value Net Asset Value
Assets
Assets held for sale	$ 562	$ 762
Investments reported at NAV	1,363	1,062
Recurring basis
Assets
Other investments	(17)	(9)
Liabilities
Other liabilities	13	18
Counterparty and cash collateral netting	17	27
Recurring basis | Quoted prices in active markets for identical assets (Level 1)
Assets
Fixed income securities, at fair value	3,074	2,061
Equity securities, at fair value	2,318	2,468
Short-term investments, at fair value	3,530	6,549
Other investments	0	0
Other assets	1	1
Assets held for sale	6,380	6,488
Total assets at fair value	15,303	17,567
Liabilities
Other liabilities	(1)	0
Liabilities held for sale	0	0
Total recurring basis liabilities	(1)	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	3,074	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	37,252	40,278
Equity securities, at fair value	426	396
Short-term investments, at fair value	2,487	223
Other investments	39	29
Other assets	0	0
Assets held for sale	25,319	23,103
Total assets at fair value	65,523	64,029
Liabilities
Other liabilities	(21)	(34)
Liabilities held for sale	(97)	(119)
Total recurring basis liabilities	(118)	(153)
Recurring basis | Significant other observable inputs (Level 2) | U.S. government and agencies
Assets
Fixed income securities, at fair value	36	45
Recurring basis | Significant other observable inputs (Level 2) | Municipal
Assets
Fixed income securities, at fair value	7,482	7,562
Recurring basis | Significant other observable inputs (Level 2) | Corporate - public
Assets
Fixed income securities, at fair value	18,818	21,885
Recurring basis | Significant other observable inputs (Level 2) | Corporate - privately placed
Assets
Fixed income securities, at fair value	9,037	9,002
Recurring basis | Significant other observable inputs (Level 2) | Foreign government
Assets
Fixed income securities, at fair value	985	958
Recurring basis | Significant other observable inputs (Level 2) | ABS
Assets
Fixed income securities, at fair value	864	794
Recurring basis | Significant other observable inputs (Level 2) | MBS
Assets
Fixed income securities, at fair value	30	32
Recurring basis | Significant unobservable inputs (Level 3)
Assets
Fixed income securities, at fair value	268	226
Equity securities, at fair value	410	304
Short-term investments, at fair value	0	35
Other investments	3	0
Other assets	0	0
Assets held for sale	170	267
Total assets at fair value	851	832
Liabilities
Other liabilities	0	0
Liabilities held for sale	(463)	(516)
Total recurring basis liabilities	(463)	(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	18	17
Recurring basis | Significant unobservable inputs (Level 3) | Corporate - public
Assets
Fixed income securities, at fair value	43	67
Recurring basis | Significant unobservable inputs (Level 3) | Corporate - privately placed
Assets
Fixed income securities, at fair value	105	63
Recurring basis | Significant unobservable inputs (Level 3) | Foreign government
Assets
Fixed income securities, at fair value		0
Fair value
Assets
Total assets at fair value	$ 81,653	$ 82,413
Assets as a percent of assets measured at fair value	100.00%	100.00%
Liabilities
Total recurring basis liabilities	$ (565)	$ (642)
Liabilities as a percent of liabilities measured at fair value	100.00%	100.00%
Fair value | Significant unobservable inputs (Level 3)
Assets
Assets held for sale	$ 3,484	$ 4,440
Liabilities
Liabilities held for sale	(8,950)	(9,424)
Fair value | Recurring basis
Assets
Fixed income securities, at fair value	40,594	42,565
Equity securities, at fair value	3,154	3,168
Short-term investments, at fair value	6,017	6,807
Other investments	25	20
Other assets	1	1
Assets held for sale	31,862	29,852
Total assets at fair value	81,653	82,413
Liabilities
Other liabilities	(9)	(16)
Liabilities held for sale	(556)	(626)
Total recurring basis liabilities	(565)	(642)
Fair value | Recurring basis | U.S. government and agencies
Assets
Fixed income securities, at fair value	3,110	2,106
Fair value | Recurring basis | Municipal
Assets
Fixed income securities, at fair value	7,500	7,579
Fair value | Recurring basis | Corporate - public
Assets
Fixed income securities, at fair value	18,861	21,952
Fair value | Recurring basis | Corporate - privately placed
Assets
Fixed income securities, at fair value	9,142	9,065
Fair value | Recurring basis | Foreign government
Assets
Fixed income securities, at fair value	985	958
Fair value | Recurring basis | ABS
Assets
Fixed income securities, at fair value	943	846
Fair value | Recurring basis | MBS
Assets
Fixed income securities, at fair value	53	59
Fair value | Recurring basis | Significant unobservable inputs (Level 3) | Foreign government
Assets
Fixed income securities, at fair value	0
Fair value | Recurring basis | Significant unobservable inputs (Level 3) | ABS
Assets
Fixed income securities, at fair value	79	52
Fair value | Recurring basis | Significant unobservable inputs (Level 3) | MBS
Assets
Fixed income securities, at fair value	$ 23	$ 27