Fair Value Measurements - Significant Unobservable Inputs (Details)	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Actuarial Assumptions, Lapses, threshold percentage	85.00%	85.00%
Actuarial Assumptions, Policyholder Deposits, threshold percentage	85.00%	85.00%
Stabilizer Products and Managed Custody Guarantee (MCG) Products [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Percentage of Plans	100.00%		100.00%
Stabilizer (Investment Only) and MCG Contracts
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Percentage of Plans	92.00%		92.00%
Stabilizer with Recordkeeping Agreements
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
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] (Deprecated 2018-01-31)
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] (Deprecated 2018-01-31)
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] (Deprecated 2018-01-31)
Fair Value Inputs, Interest Rate Implied Volatility	0.10%		0.10%
Fair Value Inputs, Nonperformance Risk	0.25%		0.38%
Fair Value Input, Actual Assumption, Partial Withdrawal	0.00%		0.00%
Fair Value Inputs, Actuarial Assumptions, Lapses	0.00%	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%	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] (Deprecated 2018-01-31)
Fair Value Inputs, Interest Rate Implied Volatility	5.80%		6.50%
Fair Value Inputs, Nonperformance Risk	0.99%		1.20%
Fair Value Input, Actual Assumption, Partial Withdrawal	0.00%		0.00%
Fair Value Inputs, Actuarial Assumptions, Lapses	50.00%	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%	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] (Deprecated 2018-01-31)
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] (Deprecated 2018-01-31)
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] (Deprecated 2018-01-31)
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] (Deprecated 2018-01-31)
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] (Deprecated 2018-01-31)
Fair Value Inputs, Nonperformance Risk	0.25%		0.38%
Fair Value Input, Actual Assumption, Partial Withdrawal	0.00%		0.00%
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] (Deprecated 2018-01-31)
Fair Value Inputs, Nonperformance Risk	0.99%		1.20%
Fair Value Input, Actual Assumption, Partial Withdrawal	7.00%		7.00%
Fair Value Inputs, Actuarial Assumptions, Lapses	56.00%		42.00%