Fair Value of Assets and Liabilities Fair Value of Assets and Liabilities (Quantitative Information for Level 3 Inputs) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2018	Dec. 31, 2017
Corporate Debt Securities [Member] \| Market Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	$ 183,680	$ 173,579
Corporate Debt Securities [Member] \| Weighted Average [Member] \| Market Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount for Lack of Marketability	1.00%	1.00%
Fair Value Inputs, Earnings before Interest, Taxes, Depreciation, and Amortization Multiple	6.6
Corporate Debt Securities [Member] \| Maximum [Member] \| Market Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount for Lack of Marketability	2.00%	2.00%
Fair Value Inputs, Earnings before Interest, Taxes, Depreciation, and Amortization Multiple	7.5
Corporate Debt Securities [Member] \| Minimum [Member] \| Market Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount for Lack of Marketability	0.00%	0.00%
Fair Value Inputs, Earnings before Interest, Taxes, Depreciation, and Amortization Multiple	5.9
Longevity Swap [Member] \| Income Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	$ 44,011	$ 40,659
Longevity Swap [Member] \| Weighted Average [Member] \| Income Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Long-Duration Contracts, Assumptions by Product and Guarantee, Mortality Rate, Low End	2.00%	2.00%
Fair Value Inputs Mortality Improvement	3.00%	3.00%
Longevity Swap [Member] \| Maximum [Member] \| Income Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Long-Duration Contracts, Assumptions by Product and Guarantee, Mortality Rate, Low End	100.00%	100.00%
Fair Value Inputs Mortality Improvement	10.00%	10.00%
Longevity Swap [Member] \| Minimum [Member] \| Income Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Long-Duration Contracts, Assumptions by Product and Guarantee, Mortality Rate, Low End	0.00%	0.00%
Fair Value Inputs Mortality Improvement	(10.00%)	(10.00%)
US Government Agencies Debt Securities [Member] \| Weighted Average [Member] \| Market Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount for Lack of Marketability	1.00%	1.00%
US Government Agencies Debt Securities [Member] \| Maximum [Member] \| Market Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount for Lack of Marketability	1.00%	1.00%
US Government Agencies Debt Securities [Member] \| Minimum [Member] \| Market Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount for Lack of Marketability	0.00%	0.00%
US States and Political Subdivisions Debt Securities [Member] \| Market Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	$ 4,452	$ 4,616
US States and Political Subdivisions Debt Securities [Member] \| Weighted Average [Member] \| Market Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount for Lack of Marketability	1.00%	1.00%
US States and Political Subdivisions Debt Securities [Member] \| Maximum [Member] \| Market Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount for Lack of Marketability	1.00%	1.00%
US States and Political Subdivisions Debt Securities [Member] \| Minimum [Member] \| Market Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount for Lack of Marketability	1.00%	1.00%
Equity Securities [Member] \| Market Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	$ 25,987	$ 0
Equity Securities [Member] \| Weighted Average [Member] \| Market Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Earnings before Interest, Taxes, Depreciation, and Amortization Multiple	8.0
Equity Securities [Member] \| Maximum [Member] \| Market Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Earnings before Interest, Taxes, Depreciation, and Amortization Multiple	13.1
Equity Securities [Member] \| Minimum [Member] \| Market Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Earnings before Interest, Taxes, Depreciation, and Amortization Multiple	6.9
Embedded Derivatives In Indexed Annuity [Member] \| Income Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Financial and Nonfinancial Liabilities, Fair Value Disclosure	$ 827,109	$ 861,758
Embedded Derivatives In Indexed Annuity [Member] \| Weighted Average [Member] \| Income Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Long-Duration Contracts, Assumptions by Product and Guarantee, Lapse Rate, Low End	9.00%	9.00%
Long-Duration Contracts, Assumptions by Product and Guarantee, Mortality Rate, Low End	2.00%	2.00%
Fair Value Inputs Withdrawal	3.00%	3.00%
Fair Value Inputs Option Budget Projection	2.00%	2.00%
Embedded Derivatives In Indexed Annuity [Member] \| Maximum [Member] \| Income Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Long-Duration Contracts, Assumptions by Product and Guarantee, Lapse Rate, Low End	35.00%	35.00%
Long-Duration Contracts, Assumptions by Product and Guarantee, Mortality Rate, Low End	100.00%	100.00%
Fair Value Inputs Withdrawal	5.00%	5.00%
Fair Value Inputs Option Budget Projection	4.00%	4.00%
Embedded Derivatives In Indexed Annuity [Member] \| Minimum [Member] \| Income Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Long-Duration Contracts, Assumptions by Product and Guarantee, Lapse Rate, Low End	0.00%	0.00%
Long-Duration Contracts, Assumptions by Product and Guarantee, Mortality Rate, Low End	0.00%	0.00%
Fair Value Inputs Withdrawal	0.00%	0.00%
Fair Value Inputs Option Budget Projection	2.00%	2.00%
Embedded Derivatives In Variable Annuity [Member] \| Income Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Financial and Nonfinancial Liabilities, Fair Value Disclosure	$ 137,686	$ 152,470
Embedded Derivatives In Variable Annuity [Member] \| Weighted Average [Member] \| Income Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Long-Duration Contracts, Assumptions by Product and Guarantee, Lapse Rate, Low End	5.00%	5.00%
Long-Duration Contracts, Assumptions by Product and Guarantee, Mortality Rate, Low End	1.00%	1.00%
Fair Value Inputs Withdrawal	4.00%	3.00%
Embedded Derivatives In Variable Annuity [Member] \| Maximum [Member] \| Income Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Long-Duration Contracts, Assumptions by Product and Guarantee, Lapse Rate, Low End	25.00%	25.00%
Long-Duration Contracts, Assumptions by Product and Guarantee, Mortality Rate, Low End	100.00%	100.00%
Fair Value Inputs Withdrawal	7.00%	7.00%
Embedded Derivatives In Variable Annuity [Member] \| Minimum [Member] \| Income Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Long-Duration Contracts, Assumptions by Product and Guarantee, Lapse Rate, Low End	0.00%	0.00%
Long-Duration Contracts, Assumptions by Product and Guarantee, Mortality Rate, Low End	0.00%	0.00%
Fair Value Inputs Withdrawal	0.00%	0.00%
Embedded Derivatives In Variable Annuity Contracts [Member] \| Weighted Average [Member] \| Income Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Entity Credit Risk	1.00%	1.00%
Fair Value Assumptions, Expected Volatility Rate	13.00%	8.00%
Embedded Derivatives In Variable Annuity Contracts [Member] \| Maximum [Member] \| Income Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Entity Credit Risk	5.00%	5.00%
Fair Value Assumptions, Expected Volatility Rate	27.00%	27.00%
Embedded Derivatives In Variable Annuity Contracts [Member] \| Minimum [Member] \| Income Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Entity Credit Risk	0.00%	0.00%
Fair Value Assumptions, Expected Volatility Rate	0.00%	0.00%
Mortality Swap [Member] \| Income Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Financial and Nonfinancial Liabilities, Fair Value Disclosure	$ 1,683	$ 1,683
Mortality Swap [Member] \| Weighted Average [Member] \| Income Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Long-Duration Contracts, Assumptions by Product and Guarantee, Mortality Rate, Low End	1.00%	1.00%
Mortality Swap [Member] \| Maximum [Member] \| Income Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Long-Duration Contracts, Assumptions by Product and Guarantee, Mortality Rate, Low End	100.00%	100.00%
Mortality Swap [Member] \| Minimum [Member] \| Income Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Long-Duration Contracts, Assumptions by Product and Guarantee, Mortality Rate, Low End	0.00%	0.00%
Total Return Swap [Member] \| Funds Withheld At Interest Embedded Derivatives [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	$ 135,805	$ 122,194
Total Return Swap [Member] \| Funds Withheld At Interest Embedded Derivatives [Member] \| Weighted Average [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Long-Duration Contracts, Assumptions by Product and Guarantee, Lapse Rate, Low End	9.00%	9.00%
Long-Duration Contracts, Assumptions by Product and Guarantee, Mortality Rate, Low End	2.00%	2.00%
Fair Value Inputs Withdrawal	3.00%	3.00%
Fair Value Inputs, Entity Credit Risk	1.00%	1.00%
Fair Value Inputs Credit Rate	2.00%	2.00%
Total Return Swap [Member] \| Funds Withheld At Interest Embedded Derivatives [Member] \| Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Long-Duration Contracts, Assumptions by Product and Guarantee, Lapse Rate, Low End	35.00%	35.00%
Long-Duration Contracts, Assumptions by Product and Guarantee, Mortality Rate, Low End	100.00%	100.00%
Fair Value Inputs Withdrawal	5.00%	5.00%
Fair Value Inputs, Entity Credit Risk	5.00%	5.00%
Fair Value Inputs Credit Rate	4.00%	4.00%
Total Return Swap [Member] \| Funds Withheld At Interest Embedded Derivatives [Member] \| Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Long-Duration Contracts, Assumptions by Product and Guarantee, Lapse Rate, Low End	0.00%	0.00%
Long-Duration Contracts, Assumptions by Product and Guarantee, Mortality Rate, Low End	0.00%	0.00%
Fair Value Inputs Withdrawal	0.00%	0.00%
Fair Value Inputs, Entity Credit Risk	0.00%	0.00%
Fair Value Inputs Credit Rate	2.00%	2.00%
Total Return Swap [Member] \| US Government Agencies Debt Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	$ 21,053	$ 22,511
Corporate Debt Securities [Member] \| Available-for-sale Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value, Equity, Level 1 to Level 2 Transfers, Description	0
Fair Value, Equity, Level 2 to Level 1 Transfers, Description	0

Fair Value of Assets and Liabilities Fair Value of Assets and Liabilities (Quantitative Information for Level 3 Inputs) (Details) - USD ($)
$ in Thousands