Fair Value Measurements - Significant Unobservable Inputs - Securities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	$ 36,964	$ 32,182
CMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	3,336	2,790
Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	12,804	10,973
Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	12,485	10,297
RMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	3,044	3,006
Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	1,952	2,126
Significant Unobservable Inputs (Level 3) | CMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	69	59
Significant Unobservable Inputs (Level 3) | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	520	514
Significant Unobservable Inputs (Level 3) | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	17	46
Significant Unobservable Inputs (Level 3) | RMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	1,230	1,261
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | CMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	$ 56	$ 42
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | CMBS | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Spread	9.00%	10.00%
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | CMBS | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Spread	1040.00%	1084.00%
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | CMBS | Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Spread	400.00%	405.00%
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	$ 251	$ 245
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Corporate | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Spread	103.00%	122.00%
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Corporate | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Spread	1000.00%	1302.00%
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Corporate | Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Spread	242.00%	345.00%
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	$ 17	$ 46
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Municipal | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Spread	192.00%	135.00%
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Municipal | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Spread	250.00%	286.00%
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Municipal | Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Spread	219.00%	252.00%
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | RMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	$ 1,215	$ 1,260
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | RMBS | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Spread	24.00%	16.00%
Constant prepayment rate	1.00%	0.00%
Constant default rate	0.00%	0.00%
Loss severity	0.00%	0.00%
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | RMBS | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Spread	351.00%	731.00%
Constant prepayment rate	25.00%	17.00%
Constant default rate	9.00%	11.00%
Loss severity	100.00%	100.00%
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | RMBS | Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Spread	74.00%	193.00%
Constant prepayment rate	6.00%	4.00%
Constant default rate	4.00%	5.00%
Loss severity	66.00%	75.00%