Fair Value Measurements - Significant Unobservable Inputs Securities (Details) $ in Millions	Sep. 30, 2025 USD ($)	Dec. 31, 2024 USD ($)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	$ 45,203	$ 42,567
Other Short-term Investments	4,219	4,068
ABS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	4,506	3,937
Collateralized Loan Obligations [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	3,379	3,250
CMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	2,498	2,736
Corporate
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	23,079	20,636
RMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	5,778	5,230
Significant Unobservable Inputs (Level 3)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	3,238	2,610
Other Short-term Investments	207	98
Significant Unobservable Inputs (Level 3) \| ABS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	83	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	164	116
Significant Unobservable Inputs (Level 3) \| CMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	133	167
Significant Unobservable Inputs (Level 3) \| Corporate
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	2,835	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	23	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	$ 83
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	4.76
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	7.39
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	4.92
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	$ 164
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	2.74
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	10.13
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	6.34
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	$ 132	$ 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	2.69	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	17.57	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	5.09	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	$ 2,721	$ 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	0.87	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	7.69	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	2.58	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	$ 23	$ 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	0.50	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	5.64	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	0.80	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	1.81	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	0.04	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	0.49	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	$ 207	$ 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	1.64	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	1.75	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	1.71	2.66