Fair Value Measurements - Significant Unobservable Inputs Securities (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Available-for-sale Securities, Debt Securities	$ 36,194	$ 36,964
CMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Available-for-sale Securities, Debt Securities	3,494	3,336
Corporate
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Available-for-sale Securities, Debt Securities	13,349	12,804
Municipal
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Available-for-sale Securities, Debt Securities	11,142	12,485
RMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Available-for-sale Securities, Debt Securities	3,207	3,044
Significant Unobservable Inputs (Level 3)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Available-for-sale Securities, Debt Securities	1,952	1,952
Significant Unobservable Inputs (Level 3) | CMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Available-for-sale Securities, Debt Securities	28	69
Significant Unobservable Inputs (Level 3) | Corporate
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Available-for-sale Securities, Debt Securities	559	520
Significant Unobservable Inputs (Level 3) | Municipal
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Available-for-sale Securities, Debt Securities	9	17
Significant Unobservable Inputs (Level 3) | RMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Available-for-sale Securities, Debt Securities	1,137	1,230
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | CMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Available-for-sale Securities, Debt Securities	$ 17	$ 56
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | CMBS | Minimum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Spread	9.00%	9.00%
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | CMBS | Maximum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Spread	1040.00%	1040.00%
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | CMBS | Weighted Average [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Spread	182.00%	400.00%
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Corporate
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Available-for-sale Securities, Debt Securities	$ 284	$ 251
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Corporate | Minimum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Spread	113.00%	103.00%
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Corporate | Maximum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Spread	767.00%	1000.00%
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Corporate | Weighted Average [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Spread	209.00%	242.00%
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Municipal
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Available-for-sale Securities, Debt Securities	$ 9	$ 17
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Municipal | Minimum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Spread	161.00%	192.00%
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Municipal | Maximum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Spread	161.00%	250.00%
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Municipal | Weighted Average [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Spread	161.00%	219.00%
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | RMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Available-for-sale Securities, Debt Securities	$ 1,071	$ 1,215
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | RMBS | Minimum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Spread	19.00%	24.00%
Constant prepayment rate	1.00%	1.00%
Constant default rate	0.00%	0.00%
Loss severity	0.00%	0.00%
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | RMBS | Maximum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Spread	326.00%	351.00%
Constant prepayment rate	25.00%	25.00%
Constant default rate	8.00%	9.00%
Loss severity	100.00%	100.00%
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | RMBS | Weighted Average [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Spread	69.00%	74.00%
Constant prepayment rate	7.00%	6.00%
Constant default rate	4.00%	4.00%
Loss severity	58.00%	66.00%