Investments (Analysis of Past Due Commercial Mortgage, Agricultural and Other Loans) (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Financing Receivable, Past Due [Line Items]
Recording investment gross of allowance for credit losses	$ 6,322,544	$ 6,160,410
Non-Accrual Status	11,586	1,301
Current
Financing Receivable, Past Due [Line Items]
Recording investment gross of allowance for credit losses	6,310,958	6,160,410
30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Recording investment gross of allowance for credit losses	1,301	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	10,285	0
Commercial mortgage loans
Financing Receivable, Past Due [Line Items]
Recording investment gross of allowance for credit losses	5,740,273	5,593,004
Non-Accrual Status	8,092	0
Commercial mortgage loans | Current
Financing Receivable, Past Due [Line Items]
Recording investment gross of allowance for credit losses	5,732,181	5,593,004
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	8,092	0
Agricultural Loan
Financing Receivable, Past Due [Line Items]
Recording investment gross of allowance for credit losses	575,293	562,046
Non-Accrual Status	3,494	1,301
Agricultural Loan | Current
Financing Receivable, Past Due [Line Items]
Recording investment gross of allowance for credit losses	571,799	562,046
Agricultural Loan | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Recording investment gross of allowance for credit losses	1,301	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	2,193	0
Other Collateralized Loans
Financing Receivable, Past Due [Line Items]
Recording investment gross of allowance for credit losses	6,978	5,360
Non-Accrual Status	0	0
Other Collateralized Loans | Current
Financing Receivable, Past Due [Line Items]
Recording investment gross of allowance for credit losses	6,978	5,360
Other Collateralized Loans | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Recording investment gross of allowance for credit losses	0	0
Other Collateralized Loans | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Recording investment gross of allowance for credit losses	0	0
Other Collateralized Loans | 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