Schedule III - Real Estate and Accumulated Depreciation - Reconciliation of Investment Properties (Details) - 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, beginning of year	$ 2,667,655	$ 2,458,629	$ 2,276,330
Acquisition of new properties	0	0	0
Improvements	256,496	228,162	197,680
Disposals	(39,665)	(19,136)	(15,381)
Balance, end of year	2,884,486	$ 2,667,655	$ 2,458,629
Aggregate cost of investment properties for federal income tax purpose	$ 2,500,000