Fair Value Measurements - Financial Assets and Liabilities Carried at Fair Value (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Financial assets at fair value:
Equity securities	$ 138
Debt Securities, including held for sale assets		$ 16,958
Equity securities, including held for sale assets		207
Recurring
Financial assets at fair value:
Debt Securities	9,872
Equity securities	138
Short-term investments	139
Debt Securities, including held for sale assets		16,958
Equity securities, including held for sale assets		207
Short-term investments, including held for sale assets		428
Recurring \| Forwards, swaps, options
Financial assets at fair value:
Derivative assets	231
Derivative assets, including held for sale assets		143
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities	0
Derivative liabilities, including held for sale liabilities		33
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Recurring
Financial assets at fair value:
Debt Securities	147
Equity securities	6
Short-term investments	0
Debt Securities, including held for sale assets		147
Equity securities, including held for sale assets		16
Short-term investments, including held for sale assets		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, including held for sale assets		0
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities	0
Derivative liabilities, including held for sale liabilities		0
Significant Other Observable Inputs (Level 2) \| Recurring
Financial assets at fair value:
Debt Securities	9,278
Equity securities	132
Short-term investments	139
Debt Securities, including held for sale assets		16,046
Equity securities, including held for sale assets		160
Short-term investments, including held for sale assets		428
Significant Other Observable Inputs (Level 2) \| Recurring \| Forwards, swaps, options
Financial assets at fair value:
Derivative assets	230
Derivative assets, including held for sale assets		143
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities	0
Derivative liabilities, including held for sale liabilities		33
Significant Unobservable Inputs (Level 3) \| Recurring
Financial assets at fair value:
Debt Securities	447
Equity securities	0
Short-term investments	0
Debt Securities, including held for sale assets		765
Equity securities, including held for sale assets		31
Short-term investments, including held for sale assets		0
Significant Unobservable Inputs (Level 3) \| Recurring \| Forwards, swaps, options
Financial assets at fair value:
Derivative assets	1
Derivative assets, including held for sale assets		0
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities	0
Derivative liabilities, including held for sale liabilities		0
Federal government and agency
Financial assets at fair value:
Debt Securities, including held for sale assets		387
Federal government and agency \| Recurring
Financial assets at fair value:
Debt Securities	312
Debt Securities, including held for sale assets		387
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, including held for sale assets		147
Federal government and agency \| Significant Other Observable Inputs (Level 2) \| Recurring
Financial assets at fair value:
Debt Securities	165
Debt Securities, including held for sale assets		240
Federal government and agency \| Significant Unobservable Inputs (Level 3) \| Recurring
Financial assets at fair value:
Debt Securities	0
Debt Securities, including held for sale assets		0
State and local government
Financial assets at fair value:
Debt Securities, including held for sale assets		171
State and local government \| Recurring
Financial assets at fair value:
Debt Securities	41
Debt Securities, including held for sale assets		171
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, including held for sale assets		0
State and local government \| Significant Other Observable Inputs (Level 2) \| Recurring
Financial assets at fair value:
Debt Securities	41
Debt Securities, including held for sale assets		171
State and local government \| Significant Unobservable Inputs (Level 3) \| Recurring
Financial assets at fair value:
Debt Securities	0
Debt Securities, including held for sale assets		0
Foreign government
Financial assets at fair value:
Debt Securities, including held for sale assets		2,616
Foreign government \| Recurring
Financial assets at fair value:
Debt Securities	365
Debt Securities, including held for sale assets		2,616
Foreign government \| Quoted Prices in Active Markets for Identical Assets (Level 1) \| Recurring
Financial assets at fair value:
Debt Securities	0
Debt Securities, including held for sale assets		0
Foreign government \| Significant Other Observable Inputs (Level 2) \| Recurring
Financial assets at fair value:
Debt Securities	365
Debt Securities, including held for sale assets		2,611
Foreign government \| Significant Unobservable Inputs (Level 3) \| Recurring
Financial assets at fair value:
Debt Securities	0
Debt Securities, including held for sale assets		5
Corporate
Financial assets at fair value:
Debt Securities, including held for sale assets		13,266
Corporate \| Recurring
Financial assets at fair value:
Debt Securities	8,806
Debt Securities, including held for sale assets		13,266
Corporate \| Quoted Prices in Active Markets for Identical Assets (Level 1) \| Recurring
Financial assets at fair value:
Debt Securities	0
Debt Securities, including held for sale assets		0
Corporate \| Significant Other Observable Inputs (Level 2) \| Recurring
Financial assets at fair value:
Debt Securities	8,394
Debt Securities, including held for sale assets		12,606
Corporate \| Significant Unobservable Inputs (Level 3) \| Recurring
Financial assets at fair value:
Debt Securities	412
Debt Securities, including held for sale assets		660
Mortgage and other asset-backed
Financial assets at fair value:
Debt Securities, including held for sale assets		518
Mortgage and other asset-backed \| Recurring
Financial assets at fair value:
Debt Securities	348
Debt Securities, including held for sale assets		518
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, including held for sale assets		0
Mortgage and other asset-backed \| Significant Other Observable Inputs (Level 2) \| Recurring
Financial assets at fair value:
Debt Securities	313
Debt Securities, including held for sale assets		418
Mortgage and other asset-backed \| Significant Unobservable Inputs (Level 3) \| Recurring
Financial assets at fair value:
Debt Securities	$ 35
Debt Securities, including held for sale assets		$ 100

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