Investments (Analysis of Past Due Commercial Mortgage, Agricultural and Other Loans) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Financing Receivable, Past Due [Line Items]
Current	$ 1,231,964	$ 1,211,215
Total Loans	1,231,964	1,211,215
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	1,209,381	1,155,777
Total Loans	1,209,381	1,155,777
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	22,583	55,438
Total Loans	22,583	55,438
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
Loans | 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Accruing Interest	$ 0	$ 0