Fair Value Measurements - Financial Assets and Liabilities Carried at Fair Value (Details) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Financial assets at fair value:
Equity securities	$ 46	$ 51
Recurring
Financial assets at fair value:
Equity securities	46	51
Short-term investments	324	206
Debt Securities, including held for sale assets	9,350	9,855
Recurring | Forwards, swaps, options
Financial assets at fair value:
Derivative assets	161	132
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities	2	4
Quoted Prices in Active Markets for Identical Assets (Level 1) | Recurring
Financial assets at fair value:
Equity securities	1	4
Short-term investments	0	0
Debt Securities, including held for sale assets	162	130
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:
Equity securities	44	47
Short-term investments	324	206
Debt Securities, including held for sale assets	8,792	9,278
Significant Other Observable Inputs (Level 2) | Recurring | Forwards, swaps, options
Financial assets at fair value:
Derivative assets	161	131
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities	2	4
Significant Unobservable Inputs (Level 3) | Recurring
Financial assets at fair value:
Equity securities	1	0
Short-term investments	0	0
Debt Securities, including held for sale assets	396	447
Significant Unobservable Inputs (Level 3) | Recurring | Forwards, swaps, options
Financial assets at fair value:
Derivative assets	0	1
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities	0	0
Federal government and agency | Recurring
Financial assets at fair value:
Debt Securities, including held for sale assets	296	267
Federal government and agency | Quoted Prices in Active Markets for Identical Assets (Level 1) | Recurring
Financial assets at fair value:
Debt Securities, including held for sale assets	162	130
Federal government and agency | Significant Other Observable Inputs (Level 2) | Recurring
Financial assets at fair value:
Debt Securities, including held for sale assets	134	137
Federal government and agency | Significant Unobservable Inputs (Level 3) | Recurring
Financial assets at fair value:
Debt Securities, including held for sale assets	0	0
State and local government | Recurring
Financial assets at fair value:
Debt Securities, including held for sale assets	37	38
State and local government | Quoted Prices in Active Markets for Identical Assets (Level 1) | Recurring
Financial assets at fair value:
Debt Securities, including held for sale assets	0	0
State and local government | Significant Other Observable Inputs (Level 2) | Recurring
Financial assets at fair value:
Debt Securities, including held for sale assets	37	38
State and local government | Significant Unobservable Inputs (Level 3) | Recurring
Financial assets at fair value:
Debt Securities, including held for sale assets	0	0
Foreign government | Recurring
Financial assets at fair value:
Debt Securities, including held for sale assets	345	352
Foreign government | Quoted Prices in Active Markets for Identical Assets (Level 1) | Recurring
Financial assets at fair value:
Debt Securities, including held for sale assets	0	0
Foreign government | Significant Other Observable Inputs (Level 2) | Recurring
Financial assets at fair value:
Debt Securities, including held for sale assets	345	352
Foreign government | Significant Unobservable Inputs (Level 3) | Recurring
Financial assets at fair value:
Debt Securities, including held for sale assets	0	0
Corporate | Recurring
Financial assets at fair value:
Debt Securities, including held for sale assets	8,320	8,833
Corporate | Quoted Prices in Active Markets for Identical Assets (Level 1) | Recurring
Financial assets at fair value:
Debt Securities, including held for sale assets	0	0
Corporate | Significant Other Observable Inputs (Level 2) | Recurring
Financial assets at fair value:
Debt Securities, including held for sale assets	7,968	8,432
Corporate | Significant Unobservable Inputs (Level 3) | Recurring
Financial assets at fair value:
Debt Securities, including held for sale assets	352	401
Mortgage and other asset-backed | Recurring
Financial assets at fair value:
Debt Securities, including held for sale assets	352	365
Mortgage and other asset-backed | Quoted Prices in Active Markets for Identical Assets (Level 1) | Recurring
Financial assets at fair value:
Debt Securities, including held for sale assets	0	0
Mortgage and other asset-backed | Significant Other Observable Inputs (Level 2) | Recurring
Financial assets at fair value:
Debt Securities, including held for sale assets	308	319
Mortgage and other asset-backed | Significant Unobservable Inputs (Level 3) | Recurring
Financial assets at fair value:
Debt Securities, including held for sale assets	$ 44	$ 46