Fair Value Measurements - Significant Unobservable Inputs Securities (Details) $ in Millions	Jun. 30, 2024 USD ($)	Dec. 31, 2023 USD ($)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	$ 40,814	$ 39,818
Other Short-term Investments	3,701	3,850
Collateralized Loan Obligations [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	3,514	3,090
CMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	2,942	3,125
Corporate
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	19,493	17,866
RMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	4,787	4,287
Significant Unobservable Inputs (Level 3)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	2,799	2,237
Other Short-term Investments	25	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	500	113
Significant Unobservable Inputs (Level 3) \| CMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	190	227
Significant Unobservable Inputs (Level 3) \| Corporate
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	2,061	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	40	36
Fair Value, Recurring [Member] \| 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	$ 97	$ 98
Fair Value, Recurring [Member] \| Significant Unobservable Inputs (Level 3) \| Collateralized Loan Obligations [Member] \| Measurement Input, Credit Spread [Member] \| Minimum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	2.75	2.68
Fair Value, Recurring [Member] \| Significant Unobservable Inputs (Level 3) \| Collateralized Loan Obligations [Member] \| Measurement Input, Credit Spread [Member] \| Maximum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	2.75	2.70
Fair Value, Recurring [Member] \| Significant Unobservable Inputs (Level 3) \| Collateralized Loan Obligations [Member] \| Measurement Input, Credit Spread [Member] \| Weighted Average [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	2.75	2.69
Fair Value, Recurring [Member] \| Significant Unobservable Inputs (Level 3) \| CMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	$ 188	$ 226
Fair Value, Recurring [Member] \| Significant Unobservable Inputs (Level 3) \| CMBS \| Measurement Input, Credit Spread [Member] \| Minimum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	1.90	3.65
Fair Value, Recurring [Member] \| Significant Unobservable Inputs (Level 3) \| CMBS \| Measurement Input, Credit Spread [Member] \| Maximum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	12.93	13.15
Fair Value, Recurring [Member] \| Significant Unobservable Inputs (Level 3) \| CMBS \| Measurement Input, Credit Spread [Member] \| Weighted Average [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	4.51	5.09
Fair Value, Recurring [Member] \| Significant Unobservable Inputs (Level 3) \| Corporate
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	$ 1,945	$ 1,741
Fair Value, Recurring [Member] \| Significant Unobservable Inputs (Level 3) \| Corporate \| Measurement Input, Credit Spread [Member] \| Minimum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.11	0.49
Fair Value, Recurring [Member] \| Significant Unobservable Inputs (Level 3) \| Corporate \| Measurement Input, Credit Spread [Member] \| Maximum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	17.21	7.43
Fair Value, Recurring [Member] \| Significant Unobservable Inputs (Level 3) \| Corporate \| Measurement Input, Credit Spread [Member] \| Weighted Average [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	3.35	3.23
Fair Value, Recurring [Member] \| Significant Unobservable Inputs (Level 3) \| RMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	$ 27	$ 36
Fair Value, Recurring [Member] \| Significant Unobservable Inputs (Level 3) \| RMBS \| Measurement Input, Credit Spread [Member] \| Minimum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.89	0.32
Fair Value, Recurring [Member] \| Significant Unobservable Inputs (Level 3) \| RMBS \| Measurement Input, Credit Spread [Member] \| Maximum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	4.18	2.98
Fair Value, Recurring [Member] \| Significant Unobservable Inputs (Level 3) \| RMBS \| Measurement Input, Credit Spread [Member] \| Weighted Average [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	1.57	1.61
Fair Value, Recurring [Member] \| Significant Unobservable Inputs (Level 3) \| RMBS \| Measurement Input, Constant Prepayment Rate [Member] \| Minimum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.01	0.01
Fair Value, Recurring [Member] \| Significant Unobservable Inputs (Level 3) \| RMBS \| Measurement Input, Constant Prepayment Rate [Member] \| Maximum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.17	0.05
Fair Value, Recurring [Member] \| Significant Unobservable Inputs (Level 3) \| RMBS \| Measurement Input, Constant Prepayment Rate [Member] \| Weighted Average [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.04	0.04
Fair Value, Recurring [Member] \| Significant Unobservable Inputs (Level 3) \| RMBS \| Measurement Input, Default Rate [Member] \| Minimum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.01	0.01
Fair Value, Recurring [Member] \| Significant Unobservable Inputs (Level 3) \| RMBS \| Measurement Input, Default Rate [Member] \| Maximum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.04	0.05
Fair Value, Recurring [Member] \| Significant Unobservable Inputs (Level 3) \| RMBS \| Measurement Input, Default Rate [Member] \| Weighted Average [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.02	0.02
Fair Value, Recurring [Member] \| Significant Unobservable Inputs (Level 3) \| RMBS \| Measurement Input, Loss Severity [Member] \| Minimum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.30	0.10
Fair Value, Recurring [Member] \| Significant Unobservable Inputs (Level 3) \| RMBS \| Measurement Input, Loss Severity [Member] \| Maximum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.64	0.70
Fair Value, Recurring [Member] \| Significant Unobservable Inputs (Level 3) \| RMBS \| Measurement Input, Loss Severity [Member] \| Weighted Average [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.38	0.41
Fair Value, Recurring [Member] \| Significant Unobservable Inputs (Level 3) \| Short-term investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Other Short-term Investments	$ 16	$ 15
Fair Value, Recurring [Member] \| Significant Unobservable Inputs (Level 3) \| Short-term investments \| Measurement Input, Credit Spread [Member] \| Minimum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Instrument, Measurement Input	9.11	5.79
Fair Value, Recurring [Member] \| Significant Unobservable Inputs (Level 3) \| Short-term investments \| Measurement Input, Credit Spread [Member] \| Maximum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Instrument, Measurement Input	9.11	12.54
Fair Value, Recurring [Member] \| Significant Unobservable Inputs (Level 3) \| Short-term investments \| Measurement Input, Credit Spread [Member] \| Weighted Average [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Instrument, Measurement Input	9.11	12.25