Segments - Reconciliation of Net Operating Income to Net Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Depreciation and amortization	$ (10,196)	$ (9,239)	$ (28,653)	$ (28,018)
General and administrative expenses	(2,367)	(2,098)	(8,092)	(7,762)
Acquisition, development and other pursuit costs	(69)	(61)	(162)	(477)
Impairment charges	(3)	(19)	(101)	(50)
Interest income	2,545	1,910	7,152	4,966
Interest expense	(4,677)	(4,253)	(13,547)	(13,282)
Loss on extinguishment of debt	(11)	0	(11)	0
Gain on real estate dispositions			0	8,087
Change in fair value of interest rate derivatives	298	87	1,256	300
Other income	65	74	233	154
Income tax benefit (provision)	120	(29)	552	(781)
Net income	5,669	10,461	18,597	24,157
Operating Segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Gross profit	19,964	19,397	59,970	61,020
Segment Reconciling Items
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Depreciation and amortization	(10,196)	(9,239)	(28,653)	(28,018)
General and administrative expenses	(2,367)	(2,098)	(8,092)	(7,762)
Acquisition, development and other pursuit costs	(69)	(61)	(162)	(477)
Impairment charges	(3)	(19)	(101)	(50)
Interest income	2,545	1,910	7,152	4,966
Interest expense	(4,677)	(4,253)	(13,547)	(13,282)
Loss on extinguishment of debt	(11)	0	(11)	0
Gain on real estate dispositions	0	4,692	0	8,087
Change in fair value of interest rate derivatives	298	87	1,256	300
Other income	65	74	233	154
Income tax benefit (provision)	$ 120	$ (29)	$ 552	$ (781)