Fair Value Measurements - Financial Assets and Liabilities Carried at Fair Value (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Financial assets at fair value:
Equity securities	$ 51	$ 138
Debt Securities, including held for sale assets	9,855
Recurring
Financial assets at fair value:
Debt Securities		9,872
Equity securities	51	138
Short-term investments	206	139
Debt Securities, including held for sale assets	9,855
Recurring | Forwards, swaps, options
Financial assets at fair value:
Derivative assets	132	231
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities	4	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Recurring
Financial assets at fair value:
Debt Securities		147
Equity securities	4	6
Short-term investments	0	0
Debt Securities, including held for sale assets	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:
Debt Securities		9,278
Equity securities	47	132
Short-term investments	206	139
Debt Securities, including held for sale assets	9,278
Significant Other Observable Inputs (Level 2) | Recurring | Forwards, swaps, options
Financial assets at fair value:
Derivative assets	131	230
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities	4	0
Significant Unobservable Inputs (Level 3) | Recurring
Financial assets at fair value:
Debt Securities		447
Equity securities	0	0
Short-term investments	0	0
Debt Securities, including held for sale assets	447
Significant Unobservable Inputs (Level 3) | Recurring | Forwards, swaps, options
Financial assets at fair value:
Derivative assets	1	1
Liabilities, Fair Value Disclosure [Abstract]
Derivative liabilities	0	0
Federal government and agency
Financial assets at fair value:
Debt Securities, including held for sale assets	267
Federal government and agency | Recurring
Financial assets at fair value:
Debt Securities		312
Debt Securities, including held for sale assets	267
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	130
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	137
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	38
State and local government | Recurring
Financial assets at fair value:
Debt Securities		41
Debt Securities, including held for sale assets	38
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	38
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	352
Foreign government | Recurring
Financial assets at fair value:
Debt Securities		365
Debt Securities, including held for sale assets	352
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	352
Foreign government | Significant Unobservable Inputs (Level 3) | Recurring
Financial assets at fair value:
Debt Securities		0
Debt Securities, including held for sale assets	0
Corporate
Financial assets at fair value:
Debt Securities, including held for sale assets	8,833
Corporate | Recurring
Financial assets at fair value:
Debt Securities		8,806
Debt Securities, including held for sale assets	8,833
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	8,432
Corporate | Significant Unobservable Inputs (Level 3) | Recurring
Financial assets at fair value:
Debt Securities		412
Debt Securities, including held for sale assets	401
Mortgage and other asset-backed
Financial assets at fair value:
Debt Securities, including held for sale assets	365
Mortgage and other asset-backed | Recurring
Financial assets at fair value:
Debt Securities		348
Debt Securities, including held for sale assets	365
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	319
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	$ 46