Investments (Analysis of Past Due Commercial Mortgage and Other Loans) (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Servicing Liabilities at Fair Value [Line Items]
Current	$ 1,277,362	$ 1,234,182
Total Loans	1,277,362	1,234,182
Non-Accrual Status	0	0
30-59 Days Past Due
Servicing Liabilities at Fair Value [Line Items]
Past Due	0	0
60-89 Days Past Due
Servicing Liabilities at Fair Value [Line Items]
Past Due	0	0
90 Days or More Past Due
Servicing Liabilities at Fair Value [Line Items]
Past Due	0	0
Commercial Mortgage Loans
Servicing Liabilities at Fair Value [Line Items]
Current	1,189,430	1,149,947
Total Loans	1,189,430	1,149,947
Non-Accrual Status	0	0
Commercial Mortgage Loans | 30-59 Days Past Due
Servicing Liabilities at Fair Value [Line Items]
Past Due	0	0
Commercial Mortgage Loans | 60-89 Days Past Due
Servicing Liabilities at Fair Value [Line Items]
Past Due	0	0
Commercial Mortgage Loans | 90 Days or More Past Due
Servicing Liabilities at Fair Value [Line Items]
Past Due	0	0
Agricultural Property Loans
Servicing Liabilities at Fair Value [Line Items]
Current	87,932	84,235
Total Loans	87,932	84,235
Non-Accrual Status	0	0
Agricultural Property Loans | 30-59 Days Past Due
Servicing Liabilities at Fair Value [Line Items]
Past Due	0	0
Agricultural Property Loans | 60-89 Days Past Due
Servicing Liabilities at Fair Value [Line Items]
Past Due	0	0
Agricultural Property Loans | 90 Days or More Past Due
Servicing Liabilities at Fair Value [Line Items]
Past Due	$ 0	$ 0