Federal Income Tax Reporting - Reconciliation Between GAAP Net Income and Taxable Income, Tax Components of Distributions (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation Between GAAP Net Income and Taxable Income
Net income per books	$ (4,932)	$ 1,903	$ (17,395)	$ 4,480	$ 1,729	$ 2,458	$ 1,612	$ 2,579	$ (15,944)	$ 8,378	$ 35,014
Adjustment to book income:
Book depreciation and amortization									100,227	92,557	91,201
Tax depreciation and amortization									(62,653)	(59,171)	(53,089)
Tax basis more than book basis on assets sold									(907)	(576)	(2,739)
Straight line rent adjustment, net									(2,977)	(2,976)	(3,493)
Deferred rent, net									1,226	2	618
Non-taxable distributions									(1,289)	(970)
Other, net									6,582	(1,648)	995
Taxable income									24,265	35,596	68,507
Less: Capital gains recognized											10,360
Taxable income subject to distribution requirement									$ 24,265	$ 35,596	$ 58,147
Tax components of the Company's common distributions paid per share
Ordinary income (in dollars per share)									$ 0.24	$ 0.39	$ 0.60
Capital gain (in dollars per share)											0.09
Return of capital (in dollars per share)									0.52	0.37	0.07
Total (in dollars per share)									$ 0.76	$ 0.76	$ 0.76
Ordinary income (as a percent)									31.74%	50.82%	79.17%
Capital gain (as a percent)											11.60%
Return of capital (as a percent)									68.26%	49.18%	9.23%
Total (as a percent)									100.00%	100.00%	100.00%
Capital gains as percentage of total distributions											11.60%
Percentage of total distributions taxed as an Unrecaptured Section 1250 gain											5.96%