Fair Value Measurements - Significant Unobservable Inputs Securities (Details) $ in Millions	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	$ 42,200	$ 42,148
CMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	4,211	4,338
Corporate
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	18,563	17,396
RMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	3,895	4,869
Significant Unobservable Inputs (Level 3)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	1,730	1,414
Significant Unobservable Inputs (Level 3) | CMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	20	9
Significant Unobservable Inputs (Level 3) | Corporate
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	1,109	732
Significant Unobservable Inputs (Level 3) | RMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	479	560
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	$ 79	$ 95
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	4.86	2.46
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	4.86	2.46
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	4.86	2.46
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		$ 1
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		0.09
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		18.32
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		1.61
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	$ 994	$ 633
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	0.78	0.93
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	10.07	7.88
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	3.23	2.36
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	$ 479	$ 560
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	0.43	0.05
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	5.28	2.33
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	1.67	0.79
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	0.11	0.11
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	0.05	0.06
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	0.07	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	0.77	0.70