Fair Value Measurements - Financial Assets and Liabilities Carried at Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Financial assets at fair value:
Equity securities	$ 53	$ 138
Recurring
Financial assets at fair value:
Debt Securities	9,539	9,872
Equity securities	53	138
Short-term investments	188	139
Recurring | Forwards, swaps, options
Financial assets at fair value:
Derivative assets	193	231
Quoted Prices in Active Markets for Identical Assets (Level 1) | Recurring
Financial assets at fair value:
Debt Securities	146	147
Equity securities	3	6
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
Significant Other Observable Inputs (Level 2) | Recurring
Financial assets at fair value:
Debt Securities	8,959	9,278
Equity securities	48	132
Short-term investments	188	139
Significant Other Observable Inputs (Level 2) | Recurring | Forwards, swaps, options
Financial assets at fair value:
Derivative assets	193	230
Significant Unobservable Inputs (Level 3) | Recurring
Financial assets at fair value:
Debt Securities	434	447
Equity securities	2	0
Short-term investments	0	0
Significant Unobservable Inputs (Level 3) | Recurring | Forwards, swaps, options
Financial assets at fair value:
Derivative assets	0	1
Federal government and agency | Recurring
Financial assets at fair value:
Debt Securities	283	312
Federal government and agency | Quoted Prices in Active Markets for Identical Assets (Level 1) | Recurring
Financial assets at fair value:
Debt Securities	146	147
Federal government and agency | Significant Other Observable Inputs (Level 2) | Recurring
Financial assets at fair value:
Debt Securities	137	165
Federal government and agency | Significant Unobservable Inputs (Level 3) | Recurring
Financial assets at fair value:
Debt Securities	0	0
State and local government | Recurring
Financial assets at fair value:
Debt Securities	38	41
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	38	41
State and local government | Significant Unobservable Inputs (Level 3) | Recurring
Financial assets at fair value:
Debt Securities	0	0
Foreign government | Recurring
Financial assets at fair value:
Debt Securities	356	365
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	356	365
Foreign government | Significant Unobservable Inputs (Level 3) | Recurring
Financial assets at fair value:
Debt Securities	0	0
Corporate | Recurring
Financial assets at fair value:
Debt Securities	8,516	8,806
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	8,125	8,394
Corporate | Significant Unobservable Inputs (Level 3) | Recurring
Financial assets at fair value:
Debt Securities	391	412
Mortgage and other asset-backed | Recurring
Financial assets at fair value:
Debt Securities	346	348
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	303	313
Mortgage and other asset-backed | Significant Unobservable Inputs (Level 3) | Recurring
Financial assets at fair value:
Debt Securities	$ 43	$ 35