Mortgage Loans on Real Estate (Mortgage Loan Portfolio Summarized by Geographic Region and Property Type) (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Mortgage Loans on Real Estate [Line Items]
Principal outstanding	$ 2,516,370	$ 2,490,619
Percent	100.00%	100.00%
East [Member]
Mortgage Loans on Real Estate [Line Items]
Principal outstanding	$ 622,220	$ 635,434
Percent	24.70%	25.50%
Middle Atlantic [Member]
Mortgage Loans on Real Estate [Line Items]
Principal outstanding	$ 154,097	$ 151,640
Percent	6.10%	6.10%
Mountain [Member]
Mortgage Loans on Real Estate [Line Items]
Principal outstanding	$ 241,197	$ 235,932
Percent	9.60%	9.50%
New England [Member]
Mortgage Loans on Real Estate [Line Items]
Principal outstanding	$ 12,610	$ 12,724
Percent	0.50%	0.50%
Pacific [Member]
Mortgage Loans on Real Estate [Line Items]
Principal outstanding	$ 403,297	$ 385,683
Percent	16.00%	15.50%
South Atlantic [Member]
Mortgage Loans on Real Estate [Line Items]
Principal outstanding	$ 546,519	$ 519,065
Percent	21.70%	20.80%
West North Central [Member]
Mortgage Loans on Real Estate [Line Items]
Principal outstanding	$ 313,751	$ 325,447
Percent	12.50%	13.10%
West South Central [Member]
Mortgage Loans on Real Estate [Line Items]
Principal outstanding	$ 222,679	$ 224,694
Percent	8.90%	9.00%
Office [Member]
Mortgage Loans on Real Estate [Line Items]
Principal outstanding	$ 297,522	$ 308,578
Percent	11.80%	12.40%
Medical Office [Member]
Mortgage Loans on Real Estate [Line Items]
Principal outstanding	$ 38,637	$ 50,780
Percent	1.60%	2.10%
Retail [Member]
Mortgage Loans on Real Estate [Line Items]
Principal outstanding	$ 910,986	$ 886,942
Percent	36.20%	35.60%
Industrial/Warehouse [Member]
Mortgage Loans on Real Estate [Line Items]
Principal outstanding	$ 709,366	$ 700,644
Percent	28.20%	28.10%
Apartment [Member]
Mortgage Loans on Real Estate [Line Items]
Principal outstanding	$ 380,630	$ 375,837
Percent	15.10%	15.10%
Mixed Use/Other [Member]
Mortgage Loans on Real Estate [Line Items]
Principal outstanding	$ 179,229	$ 167,838
Percent	7.10%	6.70%