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, 2023	Dec. 31, 2022
Assets
Fixed income securities	$ 44,103	$ 42,485
Equity securities	2,174	4,567
Short-term investments	6,722	4,173
Other investments	1,724	1,728
Total assets at fair value	53,146	51,358
Counterparty and cash collateral netting	$ 0	$ (22)
% of total assets at fair value	0.00%	0.00%
Total	$ 54,374	$ 52,604
Liabilities
% of total liabilities at fair value	(320.00%)	(420.00%)
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
Fixed income securities	$ 7,695	$ 7,898
Municipal
Assets
Fixed income securities	6,324	6,210
Foreign government
Assets
Fixed income securities	1,091	957
ABS
Assets
Fixed income securities	957	1,157
Quoted prices in active markets for identical assets (Level 1)
Assets
Total assets at fair value	$ 11,239	$ 12,325
% of total assets at fair value	21.10%	24.00%
Liabilities
% of total liabilities at fair value	100.00%	20.00%
Significant other observable inputs (Level 2)
Assets
Total assets at fair value	$ 41,288	$ 38,414
% of total assets at fair value	77.70%	74.80%
Liabilities
% of total liabilities at fair value	320.00%	500.00%
Significant unobservable inputs (Level 3)
Assets
Total assets at fair value	$ 619	$ 641
% of total assets at fair value	1.20%	1.20%
Liabilities
% of total liabilities at fair value	0.00%	0.00%
Fair Value Net Asset Value
Assets
Investments reported at NAV	$ 1,228	$ 1,246
Recurring basis
Assets
Fixed income securities	44,103	42,485
Equity securities	2,174	4,567
Short-term investments	6,722	4,173
Other investments	12	4
Other investments	0	(22)
Other assets	116	106
Total assets at fair value	53,127	51,335
Counterparty and cash collateral netting	0	(22)
Liabilities
Other liabilities	(5)	(5)
Other liabilities	16	21
Total recurring basis liabilities	(5)	(5)
Counterparty and cash collateral netting	16	21
Recurring basis \| U.S. government and agencies
Assets
Fixed income securities	7,695	7,898
Recurring basis \| Municipal
Assets
Fixed income securities	6,324	6,210
Recurring basis \| Corporate - public
Assets
Fixed income securities	20,349	18,616
Recurring basis \| Corporate - privately placed
Assets
Fixed income securities	7,687	7,647
Recurring basis \| Foreign government
Assets
Fixed income securities	1,091	957
Recurring basis \| ABS
Assets
Fixed income securities	957	1,157
Recurring basis \| Quoted prices in active markets for identical assets (Level 1)
Assets
Fixed income securities	7,678	7,878
Equity securities	1,523	3,936
Short-term investments	2,034	508
Other investments	0	0
Other assets	4	3
Total assets at fair value	11,239	12,325
Liabilities
Other liabilities	(5)	(1)
Total recurring basis liabilities	(5)	(1)
Recurring basis \| Quoted prices in active markets for identical assets (Level 1) \| U.S. government and agencies
Assets
Fixed income securities	7,678	7,878
Recurring basis \| Quoted prices in active markets for identical assets (Level 1) \| Municipal
Assets
Fixed income securities	0	0
Recurring basis \| Quoted prices in active markets for identical assets (Level 1) \| Corporate - public
Assets
Fixed income securities	0	0
Recurring basis \| Quoted prices in active markets for identical assets (Level 1) \| Corporate - privately placed
Assets
Fixed income securities	0	0
Recurring basis \| Quoted prices in active markets for identical assets (Level 1) \| Foreign government
Assets
Fixed income securities	0	0
Recurring basis \| Quoted prices in active markets for identical assets (Level 1) \| ABS
Assets
Fixed income securities	0	0
Recurring basis \| Significant other observable inputs (Level 2)
Assets
Fixed income securities	36,303	34,434
Equity securities	293	298
Short-term investments	4,682	3,659
Other investments	10	23
Other assets	0	0
Total assets at fair value	41,288	38,414
Liabilities
Other liabilities	(16)	(25)
Total recurring basis liabilities	(16)	(25)
Recurring basis \| Significant other observable inputs (Level 2) \| U.S. government and agencies
Assets
Fixed income securities	17	20
Recurring basis \| Significant other observable inputs (Level 2) \| Municipal
Assets
Fixed income securities	6,307	6,189
Recurring basis \| Significant other observable inputs (Level 2) \| Corporate - public
Assets
Fixed income securities	20,320	18,547
Recurring basis \| Significant other observable inputs (Level 2) \| Corporate - privately placed
Assets
Fixed income securities	7,638	7,592
Recurring basis \| Significant other observable inputs (Level 2) \| Foreign government
Assets
Fixed income securities	1,091	957
Recurring basis \| Significant other observable inputs (Level 2) \| ABS
Assets
Fixed income securities	930	1,129
Recurring basis \| Significant unobservable inputs (Level 3)
Assets
Fixed income securities	122	173
Equity securities	358	333
Short-term investments	6	6
Other investments	2	3
Other assets	112	103
Total assets at fair value	600	618
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	0	0
Recurring basis \| Significant unobservable inputs (Level 3) \| Municipal
Assets
Fixed income securities	17	21
Recurring basis \| Significant unobservable inputs (Level 3) \| Corporate - public
Assets
Fixed income securities	29	69
Recurring basis \| Significant unobservable inputs (Level 3) \| Corporate - privately placed
Assets
Fixed income securities	49	55
Recurring basis \| Significant unobservable inputs (Level 3) \| Foreign government
Assets
Fixed income securities	0	0
Recurring basis \| Significant unobservable inputs (Level 3) \| ABS
Assets
Fixed income securities	27	28
Non-recurring basis
Assets
Total assets at fair value	19	23
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	$ 19	$ 23

Assets as a percent of total assets measured at fair value on a recurring and non-recurring basis.

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Liabilities as a percent of total liabilities measured at fair value on a recurring and non-recurring basis.

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Other Investments, Gross Asset and Right to Reclaim Cash, Offset

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Fair value of investment other than investment in equity security, investment in debt security and equity method investment. Includes, but is not limited to, investment in certain entities that calculate net asset value per share. Example includes, but is not limited to, investment in hedge fund, venture capital fund, private equity fund, and real estate partnership or fund.

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Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Amount excluding accrued interest, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI), classified as current.

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Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

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Fair value of financial and nonfinancial obligations.

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