Mortgage Loans on Real Estate (Mortgage Loan Portfolio Summarized by Geographic Region and Property Type) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 2,952,464	$ 2,674,315	$ 2,490,619
Percent	100.00%	100.00%
East
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 586,773	$ 548,067
Percent	19.90%	20.50%
Middle Atlantic
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 168,969	$ 163,485
Percent	5.70%	6.10%
Mountain
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 357,642	$ 308,486
Percent	12.10%	11.50%
New England
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 9,418	$ 12,265
Percent	0.30%	0.50%
Pacific
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 521,363	$ 466,030
Percent	17.70%	17.40%
South Atlantic
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 694,599	$ 609,736
Percent	23.50%	22.80%
West North Central
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 291,890	$ 324,808
Percent	9.90%	12.20%
West South Central
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 321,810	$ 241,438
Percent	10.90%	9.00%
Office
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 268,932	$ 283,926
Percent	9.10%	10.60%
Medical Office
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 33,467	$ 34,338
Percent	1.10%	1.30%
Retail
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 1,091,627	$ 1,040,028
Percent	37.00%	38.90%
Industrial/Warehouse
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 762,887	$ 677,770
Percent	25.80%	25.30%
Apartment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 600,638	$ 462,897
Percent	20.30%	17.30%
Agricultural
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 25,000	$ 0
Percent	0.90%	0.00%
Mixed Use/Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal outstanding	$ 169,913	$ 175,356
Percent	5.80%	6.60%