Investments (Analysis of Past Due Commercial Mortgage, Agricultural and Other Loans) (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Financing Receivable, Past Due [Line Items]
Recording investment gross of allowance for credit losses	$ 5,519,969	$ 4,948,943
Non-Accrual Status	0	0
Current
Financing Receivable, Past Due [Line Items]
Recording investment gross of allowance for credit losses	5,519,969	4,947,614
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	1,329
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	5,005,051	4,481,925
Non-Accrual Status	0	0
Commercial mortgage loans | Current
Financing Receivable, Past Due [Line Items]
Recording investment gross of allowance for credit losses	5,005,051	4,481,925
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	514,918	467,018
Non-Accrual Status	0	0
Agricultural Loan | Current
Financing Receivable, Past Due [Line Items]
Recording investment gross of allowance for credit losses	514,918	465,689
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	1,329
Agricultural Loan | 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Recording investment gross of allowance for credit losses	0	0
Loans | 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Accruing Interest	$ 0	$ 0