Mortgage Loans on Real Estate (Commercial Mortgage Loan Portfolio Summarized by Geographic Region and Property Type) (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accrued interest receivable	$ 20,600	$ 16,600
Commercial Mortgage Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 3,459,879	$ 3,580,154
Percent	100.00%	100.00%
Commercial Mortgage Loans | East
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 676,448	$ 699,741
Percent	19.60%	19.50%
Commercial Mortgage Loans | Middle Atlantic
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 294,619	$ 281,971
Percent	8.50%	7.90%
Commercial Mortgage Loans | Mountain
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 373,129	$ 391,025
Percent	10.80%	10.90%
Commercial Mortgage Loans | New England
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 24,475	$ 24,774
Percent	0.70%	0.70%
Commercial Mortgage Loans | Pacific
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 650,256	$ 659,743
Percent	18.80%	18.40%
Commercial Mortgage Loans | South Atlantic
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 779,690	$ 832,739
Percent	22.50%	23.30%
Commercial Mortgage Loans | West North Central
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 263,600	$ 266,050
Percent	7.60%	7.40%
Commercial Mortgage Loans | West South Central
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 397,662	$ 424,111
Percent	11.50%	11.90%
Commercial Mortgage Loans | Office
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 292,968	$ 297,065
Percent	8.50%	8.30%
Commercial Mortgage Loans | Medical Office
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 17,718	$ 20,584
Percent	0.50%	0.60%
Commercial Mortgage Loans | Retail
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 1,116,376	$ 1,187,484
Percent	32.30%	33.20%
Commercial Mortgage Loans | Industrial/Warehouse
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 911,293	$ 929,325
Percent	26.30%	25.90%
Commercial Mortgage Loans | Apartment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 948,161	$ 939,084
Percent	27.40%	26.20%
Commercial Mortgage Loans | Mixed Use/Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 173,363	$ 206,612
Percent	5.00%	5.80%
Agricultural
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 277,427	$ 245,807
Residential Mortgage Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 573,638	$ 366,320