Mortgage Loans on Real Estate (Mortgage Loan Portfolio Summarized by Geographic Region and Property Type) (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Mortgage Loans on Real Estate [Line Items]
Principal outstanding	$ 2,427,905	$ 2,449,909
Percent	100.00%	100.00%
Office [Member]
Mortgage Loans on Real Estate [Line Items]
Principal outstanding	$ 321,297	$ 396,154
Percent	13.20%	16.20%
Medical Office [Member]
Mortgage Loans on Real Estate [Line Items]
Principal outstanding	$ 54,129	$ 77,438
Percent	2.20%	3.20%
Retail [Member]
Mortgage Loans on Real Estate [Line Items]
Principal outstanding	$ 825,490	$ 790,158
Percent	34.00%	32.20%
Industrial/Warehouse [Member]
Mortgage Loans on Real Estate [Line Items]
Principal outstanding	$ 677,342	$ 686,400
Percent	27.90%	28.00%
Hotel [Member]
Mortgage Loans on Real Estate [Line Items]
Principal outstanding	$ 1,142	$ 3,361
Percent	0.10%	0.10%
Apartment [Member]
Mortgage Loans on Real Estate [Line Items]
Principal outstanding	$ 376,189	$ 352,971
Percent	15.50%	14.40%
Mixed Use/Other [Member]
Mortgage Loans on Real Estate [Line Items]
Principal outstanding	$ 172,316	$ 143,427
Percent	7.10%	5.90%
East [Member]
Mortgage Loans on Real Estate [Line Items]
Principal outstanding	$ 652,230	$ 698,113
Percent	26.90%	28.50%
Middle Atlantic [Member]
Mortgage Loans on Real Estate [Line Items]
Principal outstanding	$ 155,249	$ 160,261
Percent	6.40%	6.60%
Mountain [Member]
Mortgage Loans on Real Estate [Line Items]
Principal outstanding	$ 231,561	$ 252,442
Percent	9.50%	10.30%
New England [Member]
Mortgage Loans on Real Estate [Line Items]
Principal outstanding	$ 12,834	$ 13,161
Percent	0.50%	0.50%
Pacific [Member]
Mortgage Loans on Real Estate [Line Items]
Principal outstanding	$ 387,212	$ 355,268
Percent	16.00%	14.50%
South Atlantic [Member]
Mortgage Loans on Real Estate [Line Items]
Principal outstanding	$ 474,238	$ 456,227
Percent	19.50%	18.60%
West North Central [Member]
Mortgage Loans on Real Estate [Line Items]
Principal outstanding	$ 319,341	$ 313,120
Percent	13.20%	12.80%
West South Central [Member]
Mortgage Loans on Real Estate [Line Items]
Principal outstanding	$ 195,240	$ 201,317
Percent	8.00%	8.20%