Fair Value Measurements - Significant Unobservable Inputs Securities (Details) $ in Millions	Sep. 30, 2023 USD ($)	Dec. 31, 2022 USD ($)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	$ 37,962	$ 36,231
Other Short-term Investments	3,300	3,859
Collateralized Loan Obligations [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	3,043	2,941
CMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	3,124	3,368
Corporate
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	16,651	15,233
RMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	3,827	3,708
Significant Unobservable Inputs (Level 3)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	2,088	2,051
Other Short-term Investments	185	193
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	115
Significant Unobservable Inputs (Level 3) | CMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	226	222
Significant Unobservable Inputs (Level 3) | Corporate
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	1,655	1,589
Significant Unobservable Inputs (Level 3) | RMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	64	95
Fair Value, Recurring [Member] | Significant Unobservable Inputs (Level 3)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Other Short-term Investments	$ 152
Fair Value, Recurring [Member] | Significant Unobservable Inputs (Level 3) | Measurement Input, Credit Spread [Member] | Minimum | Short-term investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Instrument, Measurement Input	2.44
Fair Value, Recurring [Member] | Significant Unobservable Inputs (Level 3) | Measurement Input, Credit Spread [Member] | Maximum | Short-term investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Instrument, Measurement Input	5.26
Fair Value, Recurring [Member] | Significant Unobservable Inputs (Level 3) | Measurement Input, Credit Spread [Member] | Weighted Average [Member] | Short-term investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Instrument, Measurement Input	2.48
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	$ 82	$ 115
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	2.86	3.37
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	2.88	3.37
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	2.87	3.37
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	$ 224	$ 219
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	3.68	4.19
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	19.04	13.07
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	5.13	5.27
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	$ 1,610	$ 1,541
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	0.20	0.77
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	8.78	6.42
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	3.52	3.60
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	$ 43	$ 65
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	0.45	0.62
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	2.67	2.49
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	1.32	1.60
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	0.07	0.10
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	0.07
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	0.05	0.04
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	0.10	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	0.76	1
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	0.41	0.38