Schedule III - Real Estate and Accumulated Depreciation (Details) - Reconciliation of Real Estate - Owned Subject to Operating Leases - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
SEC Schedule III, Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]
Balance	$ 2,687,018	$ 2,296,617	$ 1,785,898
Property improvements during the year	54,481	41,359	28,104
Properties acquired during the year	374,004	354,035	485,853
Assets written off during the year	(6,106)	(4,993)	(3,238)
Balance	$ 3,109,397	$ 2,687,018	$ 2,296,617