Schedule III - Real Estate and Accumulated Depreciation - Reconciliation of Investment Properties (Details) - 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, beginning of year	$ 2,458,629	$ 2,276,330	$ 2,139,863
Acquisition of new properties	0	0	0
Improvements	228,162	197,680	156,754
Disposals	(19,136)	(15,381)	(20,287)
Balance, end of year	2,667,655	$ 2,458,629	$ 2,276,330
Aggregate cost of investment properties for federal income tax purpose	$ 2,305,283