Selected Quarterly Financial Data (Unaudited) - Summary Certain Selected Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Rental revenues	$ 30,731	$ 28,930	$ 28,598	$ 28,699	$ 27,654	$ 27,096	$ 26,755	$ 27,232	$ 116,958	$ 108,737	$ 99,355
Net operating income	21,114	19,964	19,908	20,098	19,211	19,397	20,645	20,978
Net income	4,895	5,669	5,945	6,983	5,768	10,461	4,943	8,753	$ 23,492	$ 29,925	$ 42,755
Net income attributable to stockholders	$ 3,642	$ 4,202	$ 4,319	$ 5,040	$ 4,152	$ 7,488	$ 3,471	$ 5,936
Net income per share: basic and diluted (in dollars per share)	$ 0.07	$ 0.09	$ 0.09	$ 0.11	$ 0.09	$ 0.17	$ 0.08	$ 0.16	$ 0.36	$ 0.50	$ 0.85
General contracting and real estate services revenues
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
General contracting and real estate services revenues	$ 12,705	$ 19,950	$ 20,654	$ 23,050	$ 32,643	$ 41,201	$ 56,671	$ 63,519	$ 76,359	$ 194,034	$ 159,030