Investments (Analysis of Past Due Commercial Mortgage, Agricultural and Other Loans) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Financing Receivable, Past Due [Line Items]
Current	$ 1,801,799	$ 1,773,152
Total Loans	1,801,799	1,773,152
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	1,705,536	1,668,578
Total Loans	1,705,536	1,668,578
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	96,263	104,574
Total Loans	96,263	104,574
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