Mortgage Loans on Real Estate (Commercial Mortgage Loan Portfolio Summarized by Geographic Region and Property Type) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Commercial Mortgage Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 3,580,154	$ 3,407,611
Percent	100.00%	100.00%
Commercial Mortgage Loans | East
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 699,741	$ 645,991
Percent	19.50%	19.00%
Commercial Mortgage Loans | Middle Atlantic
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 281,971	$ 284,597
Percent	7.90%	8.40%
Commercial Mortgage Loans | Mountain
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 391,025	$ 389,892
Percent	10.90%	11.40%
Commercial Mortgage Loans | New England
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 24,774	$ 9,152
Percent	0.70%	0.30%
Commercial Mortgage Loans | Pacific
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 659,743	$ 618,336
Percent	18.40%	18.10%
Commercial Mortgage Loans | South Atlantic
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 832,739	$ 751,199
Percent	23.30%	22.00%
Commercial Mortgage Loans | West North Central
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 266,050	$ 288,413
Percent	7.40%	8.50%
Commercial Mortgage Loans | West South Central
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 424,111	$ 420,031
Percent	11.90%	12.30%
Commercial Mortgage Loans | Office
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 297,065	$ 250,287
Percent	8.30%	7.30%
Commercial Mortgage Loans | Medical Office
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 20,584	$ 29,990
Percent	0.60%	0.90%
Commercial Mortgage Loans | Retail
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 1,187,484	$ 1,225,670
Percent	33.20%	36.00%
Commercial Mortgage Loans | Industrial/Warehouse
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 929,325	$ 896,558
Percent	25.90%	26.30%
Commercial Mortgage Loans | Apartment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 939,084	$ 858,679
Percent	26.20%	25.20%
Commercial Mortgage Loans | Mixed Use/Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 206,612	$ 146,427
Percent	5.80%	4.30%
Agricultural Mortgage Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 245,807	$ 51,303
Residential Mortgage Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	366,320	$ 0
Commercial Mortgage Loans, Agricultural Mortgage Loans and Residential Mortgage Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accrued interest receivable	$ 16,600