Investments (Analysis of Past Due Commercial Mortgage, Agricultural and Other Loans) (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Financing Receivable, Past Due [Line Items]
Recording investment gross of allowance for credit losses	$ 2,159,318	$ 1,773,152
Non-Accrual Status	0	0
Commercial Mortgage Loans
Financing Receivable, Past Due [Line Items]
Recording investment gross of allowance for credit losses	2,053,525	1,668,578
Non-Accrual Status	0	0
Agricultural Loan
Financing Receivable, Past Due [Line Items]
Recording investment gross of allowance for credit losses	105,793	104,574
Non-Accrual Status	0	0
Current
Financing Receivable, Past Due [Line Items]
Recording investment gross of allowance for credit losses	2,137,087	1,773,152
Current | Commercial Mortgage Loans
Financing Receivable, Past Due [Line Items]
Recording investment gross of allowance for credit losses	2,053,525	1,668,578
Current | Agricultural Loan
Financing Receivable, Past Due [Line Items]
Recording investment gross of allowance for credit losses	83,562	104,574
30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Recording investment gross of allowance for credit losses	22,231	0
30-59 Days Past Due | Commercial Mortgage Loans
Financing Receivable, Past Due [Line Items]
Recording investment gross of allowance for credit losses	0	0
30-59 Days Past Due | Agricultural Loan
Financing Receivable, Past Due [Line Items]
Recording investment gross of allowance for credit losses	22,231	0
60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Recording investment gross of allowance for credit losses	0	0
60-89 Days Past Due | Commercial Mortgage Loans
Financing Receivable, Past Due [Line Items]
Recording investment gross of allowance for credit losses	0	0
60-89 Days Past Due | Agricultural Loan
Financing Receivable, Past Due [Line Items]
Recording investment gross of allowance for credit losses	0	0
90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Recording investment gross of allowance for credit losses	0	0
90 Days or More Past Due | Commercial Mortgage Loans
Financing Receivable, Past Due [Line Items]
Recording investment gross of allowance for credit losses	0	0
90 Days or More Past Due | Agricultural Loan
Financing Receivable, Past Due [Line Items]
Recording investment gross of allowance for credit losses	0	0
Commercial Mortgage Loans
Financing Receivable, Past Due [Line Items]
Recording investment gross of allowance for credit losses	2,053,525	1,668,578
Loans | 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Accruing Interest	$ 0	$ 0