Fair Value Measurements - Significant Unobservable Inputs (Details)	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Actuarial Assumptions, Lapses, threshold percentage	92.00%	92.00%
Actuarial Assumptions, Policyholder Deposits, threshold percentage	8.00%	8.00%
Stabilizer Products and Managed Custody Guarantee (MCG) Products [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Percentage of Plans	100.00%		100.00%
Stabilizer (Investment Only) and MCG Contracts
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Percentage of Plans	92.00%		92.00%
Stabilizer with Recordkeeping Agreements
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Percentage of Plans	8.00%		8.00%
Derivative Financial Instruments, Liabilities | FIA | Market Approach Valuation Technique | Minimum
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Fair Value Inputs, Interest Rate Implied Volatility	0.00%	0.00%
Fair Value Inputs, Actuarial Assumptions, Policyholder Deposits	0.00%	0.00%
Derivative Financial Instruments, Liabilities | FIA | Market Approach Valuation Technique | Maximum
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Fair Value Inputs, Interest Rate Implied Volatility	0.00%	0.00%
Fair Value Inputs, Actuarial Assumptions, Policyholder Deposits	0.00%	0.00%
Derivative Financial Instruments, Liabilities | Stabilizer Products and Managed Custody Guarantee (MCG) Products [Member] | Market Approach Valuation Technique | Minimum
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Fair Value Inputs, Interest Rate Implied Volatility	0.10%	0.10%
Fair Value Inputs, Nonperformance Risk	0.28%	0.02%
Fair Value Input, Actual Assumption, Partial Withdrawal	0.00%	0.00%
Fair Value Inputs, Actuarial Assumptions, Lapses	0.00%	0.00%
Actuarial Assumptions, Lapses under percent threshold	0.00%	0.00%
Fair Value Inputs, Actuarial Assumptions, Policyholder Deposits	0.00%	0.00%
Actuarial Assumptions, Policyholder Deposits under percent threshold	0.00%	0.00%
Derivative Financial Instruments, Liabilities | Stabilizer Products and Managed Custody Guarantee (MCG) Products [Member] | Market Approach Valuation Technique | Maximum
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Fair Value Inputs, Interest Rate Implied Volatility	6.30%	6.30%
Fair Value Inputs, Nonperformance Risk	0.95%	1.10%
Fair Value Input, Actual Assumption, Partial Withdrawal	0.00%	0.00%
Fair Value Inputs, Actuarial Assumptions, Lapses	50.00%	50.00%
Actuarial Assumptions, Lapses under percent threshold	30.00%	30.00%
Fair Value Inputs, Actuarial Assumptions, Policyholder Deposits	50.00%	50.00%
Actuarial Assumptions, Policyholder Deposits under percent threshold	25.00%	25.00%
Derivative Financial Instruments, Liabilities | Stabilizer (Investment Only) and MCG Contracts | Market Approach Valuation Technique | Minimum
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Fair Value Inputs, Actuarial Assumptions, Lapses	0.00%	0.00%
Actuarial Assumptions, Lapses under percent threshold	0.00%	0.00%
Fair Value Inputs, Actuarial Assumptions, Policyholder Deposits	0.00%	0.00%
Actuarial Assumptions, Policyholder Deposits under percent threshold	0.00%	0.00%
Derivative Financial Instruments, Liabilities | Stabilizer (Investment Only) and MCG Contracts | Market Approach Valuation Technique | Maximum
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Fair Value Inputs, Actuarial Assumptions, Lapses	25.00%	25.00%
Actuarial Assumptions, Lapses under percent threshold	15.00%	15.00%
Fair Value Inputs, Actuarial Assumptions, Policyholder Deposits	30.00%	30.00%
Actuarial Assumptions, Policyholder Deposits under percent threshold	15.00%	15.00%
Derivative Financial Instruments, Liabilities | Stabilizer with Recordkeeping Agreements | Market Approach Valuation Technique | Minimum
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Fair Value Inputs, Actuarial Assumptions, Lapses	0.00%	0.00%
Actuarial Assumptions, Lapses under percent threshold	0.00%	0.00%
Fair Value Inputs, Actuarial Assumptions, Policyholder Deposits	0.00%	0.00%
Actuarial Assumptions, Policyholder Deposits under percent threshold	0.00%	0.00%
Derivative Financial Instruments, Liabilities | Stabilizer with Recordkeeping Agreements | Market Approach Valuation Technique | Maximum
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Fair Value Inputs, Actuarial Assumptions, Lapses	50.00%	50.00%
Actuarial Assumptions, Lapses under percent threshold	30.00%	30.00%
Fair Value Inputs, Actuarial Assumptions, Policyholder Deposits	50.00%	50.00%
Actuarial Assumptions, Policyholder Deposits under percent threshold	25.00%	25.00%
Investment Contracts | FIA | Market Approach Valuation Technique | Minimum
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Fair Value Inputs, Nonperformance Risk	0.28%	0.02%
Fair Value Input, Actual Assumption, Partial Withdrawal	0.00%	0.50%
Fair Value Inputs, Actuarial Assumptions, Lapses	0.00%	0.00%
Investment Contracts | FIA | Market Approach Valuation Technique | Maximum
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Fair Value Inputs, Nonperformance Risk	0.95%	1.10%
Fair Value Input, Actual Assumption, Partial Withdrawal	7.00%	7.00%
Fair Value Inputs, Actuarial Assumptions, Lapses	42.00%	42.00%