Fair Value Measurements - Significant Unobservable Inputs - Securities (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	$ 21,781	$ 22,799
RMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	1,352	1,427
US States and Political Subdivisions Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	1,238	1,266
Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	13,424	14,038
CMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	1,987	2,084
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	216	284
Fair Value, Inputs, Level 1 [Member] | RMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	0	0
Fair Value, Inputs, Level 1 [Member] | US States and Political Subdivisions Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	0	0
Fair Value, Inputs, Level 1 [Member] | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	0	0
Fair Value, Inputs, Level 1 [Member] | CMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	0	0
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	1,228	1,286
Fair Value, Inputs, Level 3 [Member] | RMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	641	692
Fair Value, Inputs, Level 3 [Member] | US States and Political Subdivisions Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	37	38
Fair Value, Inputs, Level 3 [Member] | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	447	443
Fair Value, Inputs, Level 3 [Member] | CMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	21	26
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | RMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	$ 641	$ 692
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | RMBS | Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Inputs, Counterparty Credit Risk	6.00%	24.00%
Fair Value Inputs, Probability of Default	0.00%	0.00%
Fair Value Inputs, Loss Severity	0.00%	0.00%
Fair Value Inputs, Prepayment Rate	0.00%	0.00%
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | RMBS | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Inputs, Counterparty Credit Risk	428.00%	463.00%
Fair Value Inputs, Probability of Default	7.00%	7.00%
Fair Value Inputs, Loss Severity	100.00%	100.00%
Fair Value Inputs, Prepayment Rate	25.00%	25.00%
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | RMBS | Weighted Average [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Inputs, Counterparty Credit Risk	75.00%	77.00%
Fair Value Inputs, Probability of Default	4.00%	4.00%
Fair Value Inputs, Loss Severity	60.00%	65.00%
Fair Value Inputs, Prepayment Rate	6.00%	6.00%
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | US States and Political Subdivisions Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	$ 21	$ 22
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | US States and Political Subdivisions Debt Securities [Member] | Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Inputs, Treasury Yield	199.00%	192.00%
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | US States and Political Subdivisions Debt Securities [Member] | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Inputs, Treasury Yield	240.00%	250.00%
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | US States and Political Subdivisions Debt Securities [Member] | Weighted Average [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Inputs, Treasury Yield	224.00%	228.00%
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	$ 212	$ 190
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Corporate | Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Inputs, Counterparty Credit Risk	112.00%	103.00%
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Corporate | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Inputs, Counterparty Credit Risk	874.00%	1000.00%
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Corporate | Weighted Average [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Inputs, Counterparty Credit Risk	280.00%	355.00%
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | CMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	$ 14	$ 15
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | CMBS | Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Inputs, Counterparty Credit Risk	9.00%	9.00%
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | CMBS | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Inputs, Counterparty Credit Risk	1816.00%	1816.00%
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | CMBS | Weighted Average [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Inputs, Counterparty Credit Risk	433.00%	457.00%
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	$ 20,337	$ 21,229
Fair Value, Inputs, Level 2 [Member] | RMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	711	735
Fair Value, Inputs, Level 2 [Member] | US States and Political Subdivisions Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	1,201	1,228
Fair Value, Inputs, Level 2 [Member] | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	12,977	13,595
Fair Value, Inputs, Level 2 [Member] | CMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	$ 1,966	$ 2,058
Living Benefits Required to be Fair Valued and the GMWB Reinsurance Derivative [Member] | Fair Value, Inputs, Level 3 [Member] | Withdrawal Utilization [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Measurements, Sensitivity Analysis, Description	Increase	Increase
Living Benefits Required to be Fair Valued and the GMWB Reinsurance Derivative [Member] | Fair Value, Inputs, Level 3 [Member] | Withdrawal Rates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Measurements, Sensitivity Analysis, Description	Increase	Increase
Living Benefits Required to be Fair Valued and the GMWB Reinsurance Derivative [Member] | Fair Value, Inputs, Level 3 [Member] | Lapse Rates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Measurements, Sensitivity Analysis, Description	Decrease	Decrease
Living Benefits Required to be Fair Valued and the GMWB Reinsurance Derivative [Member] | Fair Value, Inputs, Level 3 [Member] | Reset Elections [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Measurements, Sensitivity Analysis, Description	Increase	Increase
Living Benefits Required to be Fair Valued and the GMWB Reinsurance Derivative [Member] | Fair Value, Inputs, Level 3 [Member] | Minimum [Member] | Withdrawal Utilization [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Unobservable Input Range	15.00%	15.00%
Living Benefits Required to be Fair Valued and the GMWB Reinsurance Derivative [Member] | Fair Value, Inputs, Level 3 [Member] | Minimum [Member] | Withdrawal Rates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Unobservable Input Range	0.00%	0.00%
Living Benefits Required to be Fair Valued and the GMWB Reinsurance Derivative [Member] | Fair Value, Inputs, Level 3 [Member] | Minimum [Member] | Lapse Rates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Unobservable Input Range	0.00%	0.00%
Living Benefits Required to be Fair Valued and the GMWB Reinsurance Derivative [Member] | Fair Value, Inputs, Level 3 [Member] | Minimum [Member] | Reset Elections [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Unobservable Input Range	30.00%	30.00%
Living Benefits Required to be Fair Valued and the GMWB Reinsurance Derivative [Member] | Fair Value, Inputs, Level 3 [Member] | Maximum [Member] | Withdrawal Utilization [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Unobservable Input Range	100.00%	100.00%
Living Benefits Required to be Fair Valued and the GMWB Reinsurance Derivative [Member] | Fair Value, Inputs, Level 3 [Member] | Maximum [Member] | Withdrawal Rates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Unobservable Input Range	8.00%	8.00%
Living Benefits Required to be Fair Valued and the GMWB Reinsurance Derivative [Member] | Fair Value, Inputs, Level 3 [Member] | Maximum [Member] | Lapse Rates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Unobservable Input Range	40.00%	40.00%
Living Benefits Required to be Fair Valued and the GMWB Reinsurance Derivative [Member] | Fair Value, Inputs, Level 3 [Member] | Maximum [Member] | Reset Elections [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Unobservable Input Range	75.00%	75.00%
Other Contract [Member] | Fair Value, Inputs, Level 3 [Member] | Equity Volatility [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Measurements, Sensitivity Analysis, Description	Increase	Increase
Other Contract [Member] | Fair Value, Inputs, Level 3 [Member] | Minimum [Member] | Equity Volatility [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Unobservable Input Range	14.00%	7.00%
Other Contract [Member] | Fair Value, Inputs, Level 3 [Member] | Maximum [Member] | Equity Volatility [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Unobservable Input Range	30.00%	30.00%