Investments (Analysis of Past Due Commercial Mortgage, Agricultural and Other Loans) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Financing Receivable, Past Due [Line Items]
Current	$ 1,231,761	$ 1,241,653
Total Loans	1,231,761	1,241,653
Non-Accrual Status	0	0
30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	0	0
60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	0	0
90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Past Due	0	0
Commercial mortgage loans
Financing Receivable, Past Due [Line Items]
Current	1,211,826	1,219,456
Total Loans	1,211,826	1,219,456
Non-Accrual Status	0	0
Commercial mortgage loans | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	0	0
Commercial mortgage loans | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	0	0
Commercial mortgage loans | 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Past Due	0	0
Agricultural property loans
Financing Receivable, Past Due [Line Items]
Current	19,935	22,197
Total Loans	19,935	22,197
Non-Accrual Status	0	0
Agricultural property loans | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	0	0
Agricultural property loans | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	0	0
Agricultural property loans | 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Past Due	0	0
Other collateralized loans
Financing Receivable, Past Due [Line Items]
Current	0
Total Loans	0
Non-Accrual Status	0
Other collateralized loans | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	0
Other collateralized loans | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	0
Other collateralized loans | 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Past Due	0
Loans | 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Accruing Interest	$ 0	$ 0