Segments - Reconciliation of Net Operating Income to Net Income (Details) - USD ($) $ 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
Segment Reporting Information
Net operating income	$ 19,211	$ 19,397	$ 20,645	$ 20,978	$ 19,740	$ 18,393	$ 17,973	$ 17,371
Depreciation and amortization									$ (37,321)	$ (35,328)	$ (23,153)
General and administrative expenses									(10,435)	(9,552)	(8,397)
Acquisition, development and other pursuit costs									(648)	(1,563)	(1,935)
Impairment charges									(110)	(355)	(41)
Interest income									7,077	3,228	126
Interest expense									(17,439)	(16,466)	(13,333)
Loss on extinguishment of debt									(50)	(82)	(512)
Gain on real estate dispositions									8,087	30,533	18,394
Change in fair value of interest rate derivatives									1,127	(941)	(229)
Other income									131	147	119
Income tax benefit (provision)									(725)	(343)	34
Net income	$ 5,768	$ 10,461	$ 4,943	$ 8,753	$ 5,145	$ 7,946	$ 3,131	$ 26,533	29,925	42,755	31,183
Operating Segments
Segment Reporting Information
Net operating income									80,231	73,477	60,110
Segment Reconciling Items
Segment Reporting Information
Depreciation and amortization									(37,321)	(35,328)	(23,153)
General and administrative expenses									(10,435)	(9,552)	(8,397)
Acquisition, development and other pursuit costs									(648)	(1,563)	(1,935)
Impairment charges									(110)	(355)	(41)
Interest income									7,077	3,228	126
Interest expense									(17,439)	(16,466)	(13,333)
Loss on extinguishment of debt									(50)	(82)	(512)
Gain on real estate dispositions									8,087	30,533	18,394
Change in fair value of interest rate derivatives									1,127	(941)	(229)
Other income									131	147	119
Income tax benefit (provision)									$ (725)	$ (343)	$ 34