Investments (Analysis of Past Due Commercial Mortgage, Agricultural and Other Loans) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Financing Receivable, Past Due [Line Items]
Current	$ 1,504,851	$ 1,474,185
Total Loans	1,509,128	1,474,185
Non-Accrual Status	0	0
30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	4,277	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	1,423,160	1,414,139
Total Loans	1,423,160	1,414,139
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	77,756	60,046
Total Loans	82,033	60,046
Non-Accrual Status	0	0
Agricultural Property Loans | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	4,277	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	3,935
Total Loans	3,935
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