Fair Value Measurements - Significant Unobservable Inputs Securities (Details) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	$ 36,819	$ 35,652
CMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	3,568	3,552
Corporate
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	14,403	13,398
Municipal
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	10,346	10,346
RMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	3,548	3,279
Significant Unobservable Inputs (Level 3)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	1,434	1,565
Significant Unobservable Inputs (Level 3) | CMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	12	12
Significant Unobservable Inputs (Level 3) | Corporate
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	525	520
Significant Unobservable Inputs (Level 3) | Municipal
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	0	0
Significant Unobservable Inputs (Level 3) | RMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Debt Securities, Available-for-sale	771	920
Fair Value, Measurements, 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	93
Fair Value, Measurements, 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	2	2
Fair Value, Measurements, 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	294	274
Fair Value, Measurements, 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	$ 725	$ 815
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) | Measurement Input, Credit Spread [Member] | Collateralized Loan Obligations [Member] | Minimum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Servicing Asset, Measurement Input	2.56
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) | Measurement Input, Credit Spread [Member] | Collateralized Loan Obligations [Member] | Maximum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Servicing Asset, Measurement Input	2.56
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) | Measurement Input, Credit Spread [Member] | Collateralized Loan Obligations [Member] | Weighted Average [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Servicing Asset, Measurement Input	2.56
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) | Measurement Input, Credit Spread [Member] | CMBS | Minimum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Servicing Asset, Measurement Input	0.09	0.09
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) | Measurement Input, Credit Spread [Member] | CMBS | Maximum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Servicing Asset, Measurement Input	10.40	10.40
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) | Measurement Input, Credit Spread [Member] | CMBS | Weighted Average [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Servicing Asset, Measurement Input	1.80	1.82
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) | Measurement Input, Credit Spread [Member] | Corporate | Minimum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Servicing Asset, Measurement Input	1.21	1.45
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) | Measurement Input, Credit Spread [Member] | Corporate | Maximum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Servicing Asset, Measurement Input	6.56	11.75
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) | Measurement Input, Credit Spread [Member] | Corporate | Weighted Average [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Servicing Asset, Measurement Input	2.13	2.63
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) | Measurement Input, Credit Spread [Member] | RMBS | Minimum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Servicing Asset, Measurement Input	0.21	0.12
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) | Measurement Input, Credit Spread [Member] | RMBS | Maximum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Servicing Asset, Measurement Input	4.07	2.15
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) | Measurement Input, Credit Spread [Member] | RMBS | Weighted Average [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Servicing Asset, Measurement Input	0.78	0.86
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) | Measurement Input, Constant Prepayment Rate [Member] | RMBS | Minimum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Constant prepayment rate [6]	0.00%	1.00%
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) | Measurement Input, Constant Prepayment Rate [Member] | RMBS | Maximum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Constant prepayment rate [6]	16.00%	15.00%
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) | Measurement Input, Constant Prepayment Rate [Member] | RMBS | Weighted Average [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Constant prepayment rate [6]	6.00%	6.00%
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) | Measurement Input, Default Rate [Member] | RMBS | Minimum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Constant default rate [6]	1.00%	1.00%
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) | Measurement Input, Default Rate [Member] | RMBS | Maximum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Constant default rate [6]	6.00%	8.00%
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) | Measurement Input, Default Rate [Member] | RMBS | Weighted Average [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Constant default rate [6]	3.00%	3.00%
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) | Measurement Input, Loss Severity [Member] | RMBS | Minimum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Constant default rate [6]	0.00%	0.00%
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) | Measurement Input, Loss Severity [Member] | RMBS | Maximum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Constant default rate [6]	100.00%	100.00%
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) | Measurement Input, Loss Severity [Member] | RMBS | Weighted Average [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Constant default rate [6]	62.00%	61.00%