Schedule III - Real Estate and Accumulated Depreciation Schedule III - Real Estate and Accumulated Depreciation (Summary) (Details) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
SEC Schedule III, Real Estate, Gross, January 1	$ 2,472,854	$ 2,095,866	$ 1,988,849
Payments to Acquire Property, Plant, and Equipment	175,793	394,737	124,113
Change In Accounts Payable, Real Property Additions	(1,855)	(430)	1,076
Additions Through Contingent Liabilities	0	0	(2,250)
Additions Through Assumption Of Debt	18,311	0	0
Payments for Delayed Tax Exempt Exchange	1,250	0	0
Settlement of Contingent Asset Acquisition Liability	0	0	750
Settlement Of Mortgage Note By Real Estate Acquisition	0	0	1,545
Additions Through Settlement Of Mortgage Note	0	9,753	255
Additions Through Reclassification of Deposit	0	227	0
SEC Schedule III, Real Estate, Cost of Real Estate Sold	(450)	(27,299)	(13,563)
SEC Schedule III, Real Estate, Other Deductions	0	0	(2,614)
SEC Schedule III, Real Estate, Gross, December 31	2,665,903	2,472,854	2,095,866
SEC Schedule III, Real Estate Accumulated Depreciation, January 1	313,080	259,059	212,300
SEC Schedule III, Real Estate Accumulated Depreciation, Depreciation Expense	67,173	59,525	53,123
SEC Schedule III, Real Estate Accumulated Depreciation, Real Estate Sold	(51)	(5,504)	(5,096)
SEC Schedule III, Real Estate Accumulated Depreciation, Other Deductions	0	0	(1,268)
SEC Schedule III, Real Estate Accumulated Depreciation, December 31	$ 380,202	$ 313,080	$ 259,059