Mortgage Loans on Real Estate (Commercial Mortgage Loan Portfolio Summarized by Geographic Region and Property Type) (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accrued interest receivable	$ 37,000	$ 16,600
Commercial Mortgage Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 3,633,131	$ 3,580,154
Percent	100.00%	100.00%
Commercial Mortgage Loans | East
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 614,406	$ 699,741
Percent	16.90%	19.50%
Commercial Mortgage Loans | Middle Atlantic
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 293,494	$ 281,971
Percent	8.10%	7.90%
Commercial Mortgage Loans | Mountain
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 452,818	$ 391,025
Percent	12.50%	10.90%
Commercial Mortgage Loans | New England
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 60,172	$ 24,774
Percent	1.60%	0.70%
Commercial Mortgage Loans | Pacific
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 863,879	$ 659,743
Percent	23.80%	18.40%
Commercial Mortgage Loans | South Atlantic
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 785,679	$ 832,739
Percent	21.60%	23.30%
Commercial Mortgage Loans | West North Central
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 235,864	$ 266,050
Percent	6.50%	7.40%
Commercial Mortgage Loans | West South Central
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 326,819	$ 424,111
Percent	9.00%	11.90%
Commercial Mortgage Loans | Office
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 315,374	$ 297,065
Percent	8.70%	8.30%
Commercial Mortgage Loans | Medical Office
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 10,827	$ 20,584
Percent	0.30%	0.60%
Commercial Mortgage Loans | Retail
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 1,016,101	$ 1,187,484
Percent	28.00%	33.20%
Commercial Mortgage Loans | Industrial/Warehouse
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 924,779	$ 929,325
Percent	25.40%	25.90%
Commercial Mortgage Loans | Apartment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 864,580	$ 939,084
Percent	23.80%	26.20%
Commercial Mortgage Loans | Hotel
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 283,500	$ 0
Percent	7.80%	0.00%
Commercial Mortgage Loans | Mixed Use/Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 217,970	$ 206,612
Percent	6.00%	5.80%
Agricultural Mortgage Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 408,135	$ 245,807
Residential Mortgage Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 1,652,910	$ 366,320