Schedule III - Real Estate Investments (Summary of Historical Cost and Accumulated Depreciation) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property
Balance, beginning of period	$ 1,964,857	$ 1,583,153	$ 1,177,582
Disposals	(18,198)	(8,946)	(5,617)
Additions (acquisitions and improvements)	410,663	390,650	411,188
Balance, end of period	2,357,322	1,964,857	1,583,153
Accumulated depreciation
Balance, beginning of period	(317,834)	(239,936)	(180,167)
Disposals	13,970	6,761	1,377
Additions (depreciation and amortization expense)	(90,959)	(84,659)	(61,146)
Balance, end of period	$ (394,823)	$ (317,834)	$ (239,936)