Fair Value Measurements - Financial Assets and Financial Liabilities Carried at Fair Value (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Financial assets at fair value:
Debt Securities	$ 16,958
Debt Securities		$ 18,131
Equity securities	207
Recurring
Financial assets at fair value:
Debt Securities	16,958
Debt Securities		18,131
Equity securities	207
Equity securities		246
Short-term investments	428
Short-term investments		325
Recurring \| Forwards, swaps, options
Financial assets at fair value:
Derivative assets	143
Derivative assets		72
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities	33
Derivative liabilities		108
Recurring \| Swaps \| Short-term investments
Financial assets at fair value:
Derivative assets		34
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Recurring
Financial assets at fair value:
Debt Securities	147
Debt Securities		207
Equity securities	16
Equity securities		50
Short-term investments	0
Short-term investments		0
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Recurring \| Forwards, swaps, options
Financial assets at fair value:
Derivative assets	0
Derivative assets		0
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities	0
Derivative liabilities		0
Significant Other Observable Inputs (Level 2) \| Recurring
Financial assets at fair value:
Debt Securities	16,046
Debt Securities		17,101
Equity securities	160
Equity securities		165
Short-term investments	428
Short-term investments		325
Significant Other Observable Inputs (Level 2) \| Recurring \| Forwards, swaps, options
Financial assets at fair value:
Derivative assets	143
Derivative assets		72
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities	33
Derivative liabilities		108
Significant Unobservable Inputs (Level 3) \| Recurring
Financial assets at fair value:
Debt Securities	765
Debt Securities		823
Equity securities	31
Equity securities		31
Short-term investments	0
Short-term investments		0
Significant Unobservable Inputs (Level 3) \| Recurring \| Forwards, swaps, options
Financial assets at fair value:
Derivative assets	0
Derivative assets		0
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities	0
Derivative liabilities		0
Federal government and agency
Financial assets at fair value:
Debt Securities	387
Debt Securities		456
Federal government and agency \| Recurring
Financial assets at fair value:
Debt Securities	387
Debt Securities		456
Federal government and agency \| Quoted Prices in Active Markets for Identical Assets (Level 1) \| Recurring
Financial assets at fair value:
Debt Securities	147
Debt Securities		207
Federal government and agency \| Significant Other Observable Inputs (Level 2) \| Recurring
Financial assets at fair value:
Debt Securities	240
Debt Securities		249
Federal government and agency \| Significant Unobservable Inputs (Level 3) \| Recurring
Financial assets at fair value:
Debt Securities	0
Debt Securities		0
State and local government
Financial assets at fair value:
Debt Securities	171
Debt Securities		167
State and local government \| Recurring
Financial assets at fair value:
Debt Securities	171
Debt Securities		167
State and local government \| Quoted Prices in Active Markets for Identical Assets (Level 1) \| Recurring
Financial assets at fair value:
Debt Securities	0
Debt Securities		0
State and local government \| Significant Other Observable Inputs (Level 2) \| Recurring
Financial assets at fair value:
Debt Securities	171
Debt Securities		167
State and local government \| Significant Unobservable Inputs (Level 3) \| Recurring
Financial assets at fair value:
Debt Securities	0
Debt Securities		0
Foreign government
Financial assets at fair value:
Debt Securities	2,616
Debt Securities		2,511
Foreign government \| Recurring
Financial assets at fair value:
Debt Securities	2,616
Debt Securities		2,511
Foreign government \| Quoted Prices in Active Markets for Identical Assets (Level 1) \| Recurring
Financial assets at fair value:
Debt Securities	0
Debt Securities		0
Foreign government \| Significant Other Observable Inputs (Level 2) \| Recurring
Financial assets at fair value:
Debt Securities	2,611
Debt Securities		2,498
Foreign government \| Significant Unobservable Inputs (Level 3) \| Recurring
Financial assets at fair value:
Debt Securities	5
Debt Securities		13
Corporate
Financial assets at fair value:
Debt Securities	13,266
Debt Securities		14,562
Corporate \| Recurring
Financial assets at fair value:
Debt Securities	13,266
Debt Securities		14,562
Corporate \| Quoted Prices in Active Markets for Identical Assets (Level 1) \| Recurring
Financial assets at fair value:
Debt Securities	0
Debt Securities		0
Corporate \| Significant Other Observable Inputs (Level 2) \| Recurring
Financial assets at fair value:
Debt Securities	12,606
Debt Securities		13,878
Corporate \| Significant Unobservable Inputs (Level 3) \| Recurring
Financial assets at fair value:
Debt Securities	660
Debt Securities		684
Mortgage and other asset-backed
Financial assets at fair value:
Debt Securities	518
Debt Securities		435
Mortgage and other asset-backed \| Recurring
Financial assets at fair value:
Debt Securities	518
Debt Securities		435
Mortgage and other asset-backed \| Quoted Prices in Active Markets for Identical Assets (Level 1) \| Recurring
Financial assets at fair value:
Debt Securities	0
Debt Securities		0
Mortgage and other asset-backed \| Significant Other Observable Inputs (Level 2) \| Recurring
Financial assets at fair value:
Debt Securities	418
Debt Securities		309
Mortgage and other asset-backed \| Significant Unobservable Inputs (Level 3) \| Recurring
Financial assets at fair value:
Debt Securities	$ 100
Debt Securities		$ 126

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