Schedule III - Real Estate and Accumulated Depreciation - Reconciliation of Investment Properties (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
SEC Schedule III, Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]
Balance, beginning of year	$ 2,276,330	$ 2,139,863	$ 1,649,423
Acquisition of new properties	0	0	354,429
Improvements	197,680	156,754	143,315
Disposals	(15,381)	(20,287)	(7,304)
Balance, end of year	2,458,629	$ 2,276,330	$ 2,139,863
Aggregate cost of investment properties for federal income tax purpose	$ 2,117,615