Supplemental Disclosures for the Statement of Consolidated Cash Flows (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Supplemental Cash Flow Elements [Abstract]
Accrued capital expenditures and deferred lease costs	$ 8,920	$ 9,417
Change in accrued dividends and discount on dividend reinvestments	(101,800)	(30,532)
Change in accrued share repurchases as part of an announced plan	(1,277)	0
Cash and cash equivalents	8,944	9,596	$ 7,382	$ 6,992
Restricted cash and escrows	1,415	1,290	1,373	1,212
Total cash, cash equivalents, and restricted cash and escrows shown in the consolidated statement of cash flows	$ 10,359	$ 10,886	$ 8,755	$ 8,204