Fair Value Measurements - Significant Unobservable Inputs Securities (Details) $ in Millions	Jun. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	$ 44,023	$ 45,035
Collateralized Loan Obligations [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	3,100	2,780
CMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	4,095	4,484
Corporate
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	19,161	20,273
RMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	3,520	4,107
Significant Unobservable Inputs (Level 3)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	1,918	1,705
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	375	360
Significant Unobservable Inputs (Level 3) | CMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	86	77
Significant Unobservable Inputs (Level 3) | Corporate
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	1,141	881
Significant Unobservable Inputs (Level 3) | RMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	302	381
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	$ 266	$ 340
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	2.29	3.04
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	3.28	3.05
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	3.12	3.04
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	$ 66	$ 20
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	1.95	2.55
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	6.45	9.75
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	3.26	6.88
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	$ 1,051	$ 749
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	0.88	1.10
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	9.65	6.92
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	2.75	2.93
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	$ 285	$ 364
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	0.38	0.07
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	2.59	9.37
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	0.94	1.19
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
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	0.12	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	0.06	0.05
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	0.02
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	0.06	0.06
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	0.03	0.03
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	0
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	1	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	0.70	0.84