Schedule III - Real Estate and Accumulated Depreciation - Reconciliation of Investment Properties (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate [Roll Forward]
Balance, beginning of year	$ 2,884,486	$ 2,667,655	$ 2,458,629
Acquisition of new properties	0	0	0
Improvements	255,924	256,496	228,162
Disposals	(30,977)	(39,665)	(19,136)
Balance, end of year	3,109,433	$ 2,884,486	$ 2,667,655
Aggregate cost of investment properties for federal income tax purpose	$ 2,600,000