Real Estate Investments - Mortgage Loans - Early Payoffs (Details) - Mortgage Loans $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) loan	Dec. 31, 2016 USD ($) loan	Dec. 31, 2015 USD ($) loan
Early mortgage loan payoffs
Pay-offs received	$ 16,665	$ 6,036	$ 2,487
AZ/MI/PA/TX/VA
Early mortgage loan payoffs
Pay-offs received	$ 10,795
Number of Loans | loan	4
MI/TX/WA
Early mortgage loan payoffs
Pay-offs received		$ 6,036
Number of Loans | loan		9
CA/TX
Early mortgage loan payoffs
Pay-offs received			$ 2,487
Number of Loans | loan			2