Mortgage Loans on Real Estate (Mortgage Loan Portfolio Summarized by Geographic Region and Property Type) (Details) - Commercial Real Estate Portfolio Segment - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 3,689,302	$ 3,458,914
Percent	100.00%	100.00%
Accrued interest receivable	$ 12,800
East
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 705,459	$ 645,991
Percent	19.10%	18.70%
Middle Atlantic
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 290,232	$ 284,597
Percent	7.90%	8.20%
Mountain
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 393,338	$ 389,892
Percent	10.70%	11.30%
New England
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 11,284	$ 9,152
Percent	0.30%	0.30%
Pacific
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 730,262	$ 655,518
Percent	19.80%	19.00%
South Atlantic
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 809,663	$ 751,199
Percent	21.90%	21.70%
West North Central
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 304,069	$ 302,534
Percent	8.20%	8.70%
West South Central
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 444,995	$ 420,031
Percent	12.10%	12.10%
Office
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 262,446	$ 250,287
Percent	7.10%	7.30%
Medical Office
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 29,551	$ 29,990
Percent	0.80%	0.90%
Retail
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 1,222,985	$ 1,225,670
Percent	33.20%	35.40%
Industrial/Warehouse
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 938,104	$ 896,558
Percent	25.40%	25.90%
Apartment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 956,947	$ 858,679
Percent	25.90%	24.80%
Agricultural
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 95,606	$ 51,303
Percent	2.60%	1.50%
Mixed Use/Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 183,663	$ 146,427
Percent	5.00%	4.20%