Real Estate Investments - Mortgage Loans (Details) - Mortgage Loans - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Mortgage Loans
Originations and fundings under mortgage loans receivable	$ 11,913	$ 20,685	$ 67,134
Pay-offs received	(16,665)	(6,036)	(2,487)
Scheduled principal payments received	(1,202)	(2,242)	(2,321)
Net (decrease) increase in mortgage loans receivable	$ (5,954)	$ 12,407	$ 62,326
Minimum
Mortgage Loans
Interest rate for mortgage loan (as a percent)	9.40%
General amortization schedule of mortgage loans	20 years
Specified basis points for annual increase in interest rate (as a percent)	0.10%
Maximum
Mortgage Loans
Interest rate for mortgage loan (as a percent)	11.20%
General amortization schedule of mortgage loans	30 years
Specified basis points for annual increase in interest rate (as a percent)	0.25%