Segments - Reconciliation of Net Operating Income to Net Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Depreciation and amortization	$ (9,179)	$ (9,304)	$ (18,457)	$ (18,779)
General and administrative expenses	(2,764)	(2,678)	(5,725)	(5,664)
Acquisition, development and other pursuit costs	(9)	(369)	(93)	(416)
Impairment charges	(98)	(27)	(98)	(31)
Interest income	2,375	1,658	4,607	3,056
Interest expense	(4,497)	(4,494)	(8,870)	(9,029)
Gain on real estate dispositions			0	3,395
Change in fair value of interest rate derivatives	(11)	(81)	958	213
Other income	54	43	168	80
Income tax (provision) benefit	166	(450)	432	(752)
Net income	5,945	4,943	12,928	13,696
Operating Segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Gross profit	19,908	20,645	40,006	41,623
Segment Reconciling Items
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Depreciation and amortization	(9,179)	(9,304)	(18,457)	(18,779)
General and administrative expenses	(2,764)	(2,678)	(5,725)	(5,664)
Acquisition, development and other pursuit costs	(9)	(369)	(93)	(416)
Impairment charges	(98)	(27)	(98)	(31)
Interest income	2,375	1,658	4,607	3,056
Interest expense	(4,497)	(4,494)	(8,870)	(9,029)
Gain on real estate dispositions	0	0	0	3,395
Change in fair value of interest rate derivatives	(11)	(81)	958	213
Other income	54	43	168	80
Income tax (provision) benefit	$ 166	$ (450)	$ 432	$ (752)