Schedule III - Real Estate and Accumulated Depreciation (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 and Accumulated Depreciation [Line Items]
Property, Plant and Equipment, Gross	$ 2,105,722	$ 2,014,936	$ 1,946,218
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate, Other Acquisition	0	0
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	(1,095,355)	(1,004,485)	$ (912,960)
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Amount of Encumbrances	995,431
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate, Accumulated Depreciation	(1,095,355)	(1,004,485)
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate, Gross	2,105,722	2,014,936
Purchase of land interests	0	0
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate, Accumulated Depreciation, Other Addition	7,666	0
Real Estate Period Additions to Cost	94,276	70,929
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate, Cost of Investment in Real Estate Sold	(3,490)	(2,211)
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate, Accumulated Depreciation, Other Deduction	0	0
Real estate period deductions	3,490	2,211
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate, Accumulated Depreciation, Depreciation Expense	(93,049)	(92,986)
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate, Accumulated Depreciation, Investment in Real Estate Sold	2,179	1,461
Real Estate Period Deductions to Accumulated Depreciation	$ 2,179	$ 1,461