Fair Value Measurements - Financial Assets and Liabilities Carried at Fair Value (Details) - USD ($) $ in Millions	Jun. 30, 2022	Dec. 31, 2021
Financial assets at fair value:
Debt Securities	$ 13,950	$ 16,958
Equity securities	426	207
Recurring
Financial assets at fair value:
Debt Securities	13,950	16,958
Equity securities	426	207
Short-term investments	199	428
Recurring | Forwards, swaps, options
Financial assets at fair value:
Derivative assets	258	143
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities	54	33
Recurring | Swaps | Short-term investments
Financial assets at fair value:
Derivative assets
Quoted Prices in Active Markets for Identical Assets (Level 1) | Recurring
Financial assets at fair value:
Debt Securities	148	147
Equity securities	9	16
Short-term investments	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Recurring | Forwards, swaps, options
Financial assets at fair value:
Derivative assets	0	0
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities	0	0
Significant Other Observable Inputs (Level 2) | Recurring
Financial assets at fair value:
Debt Securities	13,290	16,046
Equity securities	417	160
Short-term investments	199	428
Significant Other Observable Inputs (Level 2) | Recurring | Forwards, swaps, options
Financial assets at fair value:
Derivative assets	258	143
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities	54	33
Significant Unobservable Inputs (Level 3) | Recurring
Financial assets at fair value:
Debt Securities	512	765
Equity securities	0	31
Short-term investments	0	0
Significant Unobservable Inputs (Level 3) | Recurring | Forwards, swaps, options
Financial assets at fair value:
Derivative assets	0	0
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities	0	0
Federal government and agency
Financial assets at fair value:
Debt Securities	353	387
Federal government and agency | Recurring
Financial assets at fair value:
Debt Securities	353	387
Federal government and agency | Quoted Prices in Active Markets for Identical Assets (Level 1) | Recurring
Financial assets at fair value:
Debt Securities	148	147
Federal government and agency | Significant Other Observable Inputs (Level 2) | Recurring
Financial assets at fair value:
Debt Securities	205	240
Federal government and agency | Significant Unobservable Inputs (Level 3) | Recurring
Financial assets at fair value:
Debt Securities	0	0
State and local government
Financial assets at fair value:
Debt Securities	145	171
State and local government | Recurring
Financial assets at fair value:
Debt Securities	145	171
State and local government | Quoted Prices in Active Markets for Identical Assets (Level 1) | Recurring
Financial assets at fair value:
Debt Securities	0	0
State and local government | Significant Other Observable Inputs (Level 2) | Recurring
Financial assets at fair value:
Debt Securities	145	171
State and local government | Significant Unobservable Inputs (Level 3) | Recurring
Financial assets at fair value:
Debt Securities	0	0
Foreign government
Financial assets at fair value:
Debt Securities	2,331	2,616
Foreign government | Recurring
Financial assets at fair value:
Debt Securities	2,331	2,616
Foreign government | Quoted Prices in Active Markets for Identical Assets (Level 1) | Recurring
Financial assets at fair value:
Debt Securities	0	0
Foreign government | Significant Other Observable Inputs (Level 2) | Recurring
Financial assets at fair value:
Debt Securities	2,331	2,611
Foreign government | Significant Unobservable Inputs (Level 3) | Recurring
Financial assets at fair value:
Debt Securities	0	5
Corporate
Financial assets at fair value:
Debt Securities	10,754	13,266
Corporate | Recurring
Financial assets at fair value:
Debt Securities	10,754	13,266
Corporate | Quoted Prices in Active Markets for Identical Assets (Level 1) | Recurring
Financial assets at fair value:
Debt Securities	0	0
Corporate | Significant Other Observable Inputs (Level 2) | Recurring
Financial assets at fair value:
Debt Securities	10,327	12,606
Corporate | Significant Unobservable Inputs (Level 3) | Recurring
Financial assets at fair value:
Debt Securities	427	660
Mortgage and other asset-backed
Financial assets at fair value:
Debt Securities	367	518
Mortgage and other asset-backed | Recurring
Financial assets at fair value:
Debt Securities	367	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	0
Mortgage and other asset-backed | Significant Other Observable Inputs (Level 2) | Recurring
Financial assets at fair value:
Debt Securities	282	418
Mortgage and other asset-backed | Significant Unobservable Inputs (Level 3) | Recurring
Financial assets at fair value:
Debt Securities	$ 85	$ 100