Fair Value Measurements - Significant Unobservable Inputs Securities (Details) $ in Millions	Sep. 30, 2024 USD ($)	Dec. 31, 2023 USD ($)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	$ 42,793	$ 39,818
Other Short-term Investments	3,964	3,850
Collateralized Loan Obligations [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	3,563	3,090
CMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	2,857	3,125
Corporate
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	20,558	17,866
RMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	5,123	4,287
Significant Unobservable Inputs (Level 3)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	2,815	2,237
Other Short-term Investments	9	25
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	249	113
Significant Unobservable Inputs (Level 3) \| CMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	206	227
Significant Unobservable Inputs (Level 3) \| Corporate
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	2,257	1,861
Significant Unobservable Inputs (Level 3) \| RMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	55	36
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	$ 79	$ 98
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	2.75	2.68
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	2.75	2.70
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	2.75	2.69
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	$ 204	$ 226
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	2.50	3.65
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	12.04	13.15
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	4.44	5.09
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	$ 2,141	$ 1,741
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	0.34	0.49
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	7.98	7.43
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	3.14	3.23
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	$ 50	$ 36
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	0.68	0.32
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	0.01	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	0.01	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	0.30	0.10
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	3.66	2.98
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	0.07	0.05
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	0.04	0.05
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	0.70
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	1.88	1.61
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	0.38	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	$ 3	$ 15
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	5.08	5.79
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	5.51	12.54
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	5.35	12.25