Fair Value Measurements - Significant Unobservable Inputs Securities (Details) $ in Millions	Jun. 30, 2025 USD ($)	Dec. 31, 2024 USD ($)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	$ 44,558	$ 42,567
Other Short-term Investments	3,649	4,068
ABS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	4,376	3,937
Collateralized Loan Obligations [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	3,393	3,250
CMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	2,585	2,736
Corporate
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	22,525	20,636
RMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	5,513	5,230
Significant Unobservable Inputs (Level 3)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	3,045	2,610
Other Short-term Investments	116	98
Significant Unobservable Inputs (Level 3) \| ABS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	92	22
Significant Unobservable Inputs (Level 3) \| Collateralized Loan Obligations [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	70	116
Significant Unobservable Inputs (Level 3) \| CMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	137	167
Significant Unobservable Inputs (Level 3) \| Corporate
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	2,734	2,281
Significant Unobservable Inputs (Level 3) \| RMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	12	24
Significant Unobservable Inputs (Level 3) \| Fair Value, Recurring [Member] \| ABS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	$ 84
Significant Unobservable Inputs (Level 3) \| Fair Value, Recurring [Member] \| ABS \| Minimum \| Measurement Input, Credit Spread [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	4.87
Significant Unobservable Inputs (Level 3) \| Fair Value, Recurring [Member] \| ABS \| Maximum \| Measurement Input, Credit Spread [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	7.52
Significant Unobservable Inputs (Level 3) \| Fair Value, Recurring [Member] \| ABS \| Weighted Average [Member] \| Measurement Input, Credit Spread [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	5.05
Significant Unobservable Inputs (Level 3) \| Fair Value, Recurring [Member] \| Collateralized Loan Obligations [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	$ 55
Significant Unobservable Inputs (Level 3) \| Fair Value, Recurring [Member] \| Collateralized Loan Obligations [Member] \| Minimum \| Measurement Input, Credit Spread [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	4.50
Significant Unobservable Inputs (Level 3) \| Fair Value, Recurring [Member] \| Collateralized Loan Obligations [Member] \| Maximum \| Measurement Input, Credit Spread [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	10.09
Significant Unobservable Inputs (Level 3) \| Fair Value, Recurring [Member] \| Collateralized Loan Obligations [Member] \| Weighted Average [Member] \| Measurement Input, Credit Spread [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	8.51
Significant Unobservable Inputs (Level 3) \| Fair Value, Recurring [Member] \| CMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	$ 136	$ 166
Significant Unobservable Inputs (Level 3) \| Fair Value, Recurring [Member] \| CMBS \| Minimum \| Measurement Input, Credit Spread [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	2	2
Significant Unobservable Inputs (Level 3) \| Fair Value, Recurring [Member] \| CMBS \| Maximum \| Measurement Input, Credit Spread [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	18.97	12.21
Significant Unobservable Inputs (Level 3) \| Fair Value, Recurring [Member] \| CMBS \| Weighted Average [Member] \| Measurement Input, Credit Spread [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	5.63	4.18
Significant Unobservable Inputs (Level 3) \| Fair Value, Recurring [Member] \| Corporate
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	$ 2,620	$ 2,166
Significant Unobservable Inputs (Level 3) \| Fair Value, Recurring [Member] \| Corporate \| Minimum \| Measurement Input, Credit Spread [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.84	0.81
Significant Unobservable Inputs (Level 3) \| Fair Value, Recurring [Member] \| Corporate \| Maximum \| Measurement Input, Credit Spread [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	7.37	7.94
Significant Unobservable Inputs (Level 3) \| Fair Value, Recurring [Member] \| Corporate \| Weighted Average [Member] \| Measurement Input, Credit Spread [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	2.74	2.86
Significant Unobservable Inputs (Level 3) \| Fair Value, Recurring [Member] \| RMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	$ 12	$ 19
Significant Unobservable Inputs (Level 3) \| Fair Value, Recurring [Member] \| RMBS \| Minimum \| Measurement Input, Credit Spread [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.61	1
Significant Unobservable Inputs (Level 3) \| Fair Value, Recurring [Member] \| RMBS \| Minimum \| Measurement Input, Constant Prepayment Rate [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.02	0.01
Significant Unobservable Inputs (Level 3) \| Fair Value, Recurring [Member] \| RMBS \| Minimum \| Measurement Input, Default Rate [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0	0.01
Significant Unobservable Inputs (Level 3) \| Fair Value, Recurring [Member] \| RMBS \| Minimum \| Measurement Input, Loss Severity [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.35	0.30
Significant Unobservable Inputs (Level 3) \| Fair Value, Recurring [Member] \| RMBS \| Maximum \| Measurement Input, Credit Spread [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	5.73	3.72
Significant Unobservable Inputs (Level 3) \| Fair Value, Recurring [Member] \| RMBS \| Maximum \| Measurement Input, Constant Prepayment Rate [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.05	0.06
Significant Unobservable Inputs (Level 3) \| Fair Value, Recurring [Member] \| RMBS \| Maximum \| Measurement Input, Default Rate [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.05	0.04
Significant Unobservable Inputs (Level 3) \| Fair Value, Recurring [Member] \| RMBS \| Maximum \| Measurement Input, Loss Severity [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.50	0.50
Significant Unobservable Inputs (Level 3) \| Fair Value, Recurring [Member] \| RMBS \| Weighted Average [Member] \| Measurement Input, Credit Spread [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	1.83	1.81
Significant Unobservable Inputs (Level 3) \| Fair Value, Recurring [Member] \| RMBS \| Weighted Average [Member] \| Measurement Input, Constant Prepayment Rate [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.03	0.04
Significant Unobservable Inputs (Level 3) \| Fair Value, Recurring [Member] \| RMBS \| Weighted Average [Member] \| Measurement Input, Default Rate [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.02	0.02
Significant Unobservable Inputs (Level 3) \| Fair Value, Recurring [Member] \| RMBS \| Weighted Average [Member] \| Measurement Input, Loss Severity [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.44	0.41
Significant Unobservable Inputs (Level 3) \| Fair Value, Recurring [Member] \| Short-term investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Other Short-term Investments	$ 116	$ 98
Significant Unobservable Inputs (Level 3) \| Fair Value, Recurring [Member] \| Short-term investments \| Minimum \| Measurement Input, Credit Spread [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Instrument, Measurement Input	1.56	2.66
Significant Unobservable Inputs (Level 3) \| Fair Value, Recurring [Member] \| Short-term investments \| Maximum \| Measurement Input, Credit Spread [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Instrument, Measurement Input	2.69	2.66
Significant Unobservable Inputs (Level 3) \| Fair Value, Recurring [Member] \| Short-term investments \| Weighted Average [Member] \| Measurement Input, Credit Spread [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Instrument, Measurement Input	2.20	2.66