Fair Value Measurements - Significant Unobservable Inputs Securities (Details) $ in Millions	Mar. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	$ 40,840	$ 39,818
Other Short-term Investments	2,975	3,850
Collateralized Loan Obligations [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	3,168	3,090
CMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	3,050	3,125
Corporate
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	18,657	17,866
RMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	4,473	4,287
Significant Unobservable Inputs (Level 3)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	2,433	2,237
Other Short-term Investments	26	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	184	113
Significant Unobservable Inputs (Level 3) | CMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	241	227
Significant Unobservable Inputs (Level 3) | Corporate
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	1,923	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	69	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	$ 124	$ 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	2.59	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	2.60	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	2.60	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	$ 239	$ 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	2.35	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	12.25	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	4.30	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	$ 1,806	$ 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	0.07	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	7.14	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	3.04	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	$ 69	$ 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	0.44	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	4.20	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	1.58	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	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	0.03	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	8.87	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	8.87	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	8.87	12.25