Investments (Analysis of Past Due Commercial Mortgage, Agricultural and Other Loans) (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Financing Receivable, Past Due [Line Items]
Recording investment gross of allowance for credit losses	$ 2,983,584	$ 2,838,511
Non-Accrual Status	0	0
Current
Financing Receivable, Past Due [Line Items]
Recording investment gross of allowance for credit losses	2,983,584	2,838,511
30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Recording investment gross of allowance for credit losses	0	0
60-89 Days Past Due
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
Commercial mortgage loans
Financing Receivable, Past Due [Line Items]
Recording investment gross of allowance for credit losses	2,790,072	2,666,175
Non-Accrual Status	0	0
Commercial mortgage loans | Current
Financing Receivable, Past Due [Line Items]
Recording investment gross of allowance for credit losses	2,790,072	2,666,175
Commercial mortgage loans | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Recording investment gross of allowance for credit losses	0	0
Commercial mortgage loans | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Recording investment gross of allowance for credit losses	0	0
Commercial mortgage loans | 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Recording investment gross of allowance for credit losses	0	0
Agricultural Loan
Financing Receivable, Past Due [Line Items]
Recording investment gross of allowance for credit losses	193,512	172,336
Non-Accrual Status	0	0
Agricultural Loan | Current
Financing Receivable, Past Due [Line Items]
Recording investment gross of allowance for credit losses	193,512	172,336
Agricultural Loan | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Recording investment gross of allowance for credit losses	0	0
Agricultural Loan | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Recording investment gross of allowance for credit losses	0	0
Agricultural Loan | 90 Days or More Past Due
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	2,790,072	2,666,175
Loans | 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Accruing Interest	$ 0	$ 0