Mortgage Loans on Real Estate (Mortgage Loan Portfolio Summarized by Geographic Region and Property Type) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Mortgage Loans on Real Estate [Line Items]
Principal outstanding	$ 2,900,458	$ 2,674,315
Percent	100.00%	100.00%
East
Mortgage Loans on Real Estate [Line Items]
Principal outstanding	$ 598,239	$ 548,067
Percent	20.60%	20.50%
Middle Atlantic
Mortgage Loans on Real Estate [Line Items]
Principal outstanding	$ 170,836	$ 163,485
Percent	5.90%	6.10%
Mountain
Mortgage Loans on Real Estate [Line Items]
Principal outstanding	$ 348,233	$ 308,486
Percent	12.00%	11.50%
New England
Mortgage Loans on Real Estate [Line Items]
Principal outstanding	$ 9,482	$ 12,265
Percent	0.30%	0.50%
Pacific
Mortgage Loans on Real Estate [Line Items]
Principal outstanding	$ 494,791	$ 466,030
Percent	17.10%	17.40%
South Atlantic
Mortgage Loans on Real Estate [Line Items]
Principal outstanding	$ 667,043	$ 609,736
Percent	23.00%	22.80%
West North Central
Mortgage Loans on Real Estate [Line Items]
Principal outstanding	$ 309,332	$ 324,808
Percent	10.70%	12.20%
West South Central
Mortgage Loans on Real Estate [Line Items]
Principal outstanding	$ 302,502	$ 241,438
Percent	10.40%	9.00%
Office
Mortgage Loans on Real Estate [Line Items]
Principal outstanding	$ 267,017	$ 283,926
Percent	9.20%	10.60%
Medical Office
Mortgage Loans on Real Estate [Line Items]
Principal outstanding	$ 33,950	$ 34,338
Percent	1.20%	1.30%
Retail
Mortgage Loans on Real Estate [Line Items]
Principal outstanding	$ 1,076,573	$ 1,040,028
Percent	37.10%	38.90%
Industrial/Warehouse
Mortgage Loans on Real Estate [Line Items]
Principal outstanding	$ 743,787	$ 677,770
Percent	25.70%	25.30%
Apartment
Mortgage Loans on Real Estate [Line Items]
Principal outstanding	$ 604,120	$ 462,897
Percent	20.80%	17.30%
Mixed Use/Other
Mortgage Loans on Real Estate [Line Items]
Principal outstanding	$ 175,011	$ 175,356
Percent	6.00%	6.60%