Investments Investments (Analysis of Past Due Commercial Mortgage, Agricultural, and Other Loans) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	$ 1,085,213	$ 1,151,939
Total Loans	1,085,213	1,151,939
Non-Accrual Status	0	0
30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	0
60 to 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	0
Greater than or Equal to 90 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	0
Commercial mortgage loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,017,974	1,088,616
Total Loans	1,017,974	1,088,616
Non-Accrual Status	0	0
Commercial mortgage loans | 30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	0
Commercial mortgage loans | 60 to 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	0
Commercial mortgage loans | Greater than or Equal to 90 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	0
Agricultural Property Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	67,239	63,323
Total Loans	67,239	63,323
Non-Accrual Status	0	0
Agricultural Property Loans | 30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	0
Agricultural Property Loans | 60 to 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	0
Agricultural Property Loans | Greater than or Equal to 90 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 0	$ 0