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, 2018	Dec. 31, 2017	Dec. 31, 2016
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate [Roll Forward]
Balance	$ 3,109,397	$ 2,687,018	$ 2,296,617
Property improvements during the year	40,300	54,481	41,359
Properties acquired during the year	43,387	374,004	354,035
Properties sold during the year	24,427	0	0
Assets written off during the year	(8,185)	(6,106)	(4,993)
Balance	$ 3,160,472	$ 3,109,397	$ 2,687,018