Fair Value Measurements - Financial Assets and Liabilities Carried at Fair Value (Details) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024
Financial assets at fair value:
Equity securities	$ 38	$ 37
Recurring
Financial assets at fair value:
Debt Securities	8,215
Equity securities	38	37
Short-term investments	196	170
Quoted Prices in Active Markets for Identical Assets (Level 1) | Recurring
Financial assets at fair value:
Debt Securities	104
Equity securities	1	1
Short-term investments	0	0
Significant Other Observable Inputs (Level 2) | Recurring
Financial assets at fair value:
Debt Securities	7,738
Equity securities	37	36
Short-term investments	196	170
Significant Unobservable Inputs (Level 3) | Recurring
Financial assets at fair value:
Debt Securities	373
Equity securities	0	0
Short-term investments	0	0
Federal government and agency | Recurring
Financial assets at fair value:
Debt Securities	223
Federal government and agency | Quoted Prices in Active Markets for Identical Assets (Level 1) | Recurring
Financial assets at fair value:
Debt Securities	104
Federal government and agency | Significant Other Observable Inputs (Level 2) | Recurring
Financial assets at fair value:
Debt Securities	119
Federal government and agency | Significant Unobservable Inputs (Level 3) | Recurring
Financial assets at fair value:
Debt Securities	0
State and local government | Recurring
Financial assets at fair value:
Debt Securities	25
State and local government | Quoted Prices in Active Markets for Identical Assets (Level 1) | Recurring
Financial assets at fair value:
Debt Securities	0
State and local government | Significant Other Observable Inputs (Level 2) | Recurring
Financial assets at fair value:
Debt Securities	25
State and local government | Significant Unobservable Inputs (Level 3) | Recurring
Financial assets at fair value:
Debt Securities	0
Foreign government | Recurring
Financial assets at fair value:
Debt Securities	358
Foreign government | Quoted Prices in Active Markets for Identical Assets (Level 1) | Recurring
Financial assets at fair value:
Debt Securities	0
Foreign government | Significant Other Observable Inputs (Level 2) | Recurring
Financial assets at fair value:
Debt Securities	358
Foreign government | Significant Unobservable Inputs (Level 3) | Recurring
Financial assets at fair value:
Debt Securities	0
Corporate | Recurring
Financial assets at fair value:
Debt Securities	7,280
Corporate | Quoted Prices in Active Markets for Identical Assets (Level 1) | Recurring
Financial assets at fair value:
Debt Securities	0
Corporate | Significant Other Observable Inputs (Level 2) | Recurring
Financial assets at fair value:
Debt Securities	6,951
Corporate | Significant Unobservable Inputs (Level 3) | Recurring
Financial assets at fair value:
Debt Securities	329
Mortgage and other asset-backed | Recurring
Financial assets at fair value:
Debt Securities	329
Mortgage and other asset-backed | Quoted Prices in Active Markets for Identical Assets (Level 1) | Recurring
Financial assets at fair value:
Debt Securities	0
Mortgage and other asset-backed | Significant Other Observable Inputs (Level 2) | Recurring
Financial assets at fair value:
Debt Securities	285
Mortgage and other asset-backed | Significant Unobservable Inputs (Level 3) | Recurring
Financial assets at fair value:
Debt Securities	44
Derivatives | Recurring
Financial assets at fair value:
Derivative assets	153	168
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities	2	1
Derivatives | Quoted Prices in Active Markets for Identical Assets (Level 1) | Recurring
Financial assets at fair value:
Derivative assets	0	0
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities	0	0
Derivatives | Significant Other Observable Inputs (Level 2) | Recurring
Financial assets at fair value:
Derivative assets	153	168
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities	2	1
Derivatives | Significant Unobservable Inputs (Level 3) | Recurring
Financial assets at fair value:
Derivative assets	0	0
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities	$ 0	$ 0