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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Rental revenues	$ 41,832	$ 42,220	$ 36,378	$ 30,909	$ 30,731	$ 28,930	$ 28,598	$ 28,699	$ 151,339	$ 116,958	$ 108,737
Net operating income	28,869	29,359	26,333	21,806	21,114	19,964	19,908	20,098
Net income	7,855	12,063	5,826	6,514	4,895	5,669	5,945	6,983	32,258	23,492	29,925
Net income attributable to common stockholders	$ 5,223	$ 7,079	$ 4,412	$ 4,884	$ 3,642	$ 4,202	$ 4,319	$ 5,040	$ 21,598	$ 17,203	$ 21,047
Net income per share: basic and diluted (in dollars per share)	$ 0.09	$ 0.13	$ 0.08	$ 0.10	$ 0.07	$ 0.09	$ 0.09	$ 0.11	$ 0.41	$ 0.36	$ 0.50
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	$ 39,741	$ 27,638	$ 21,444	$ 17,036	$ 12,705	$ 19,950	$ 20,654	$ 23,050	$ 105,859	$ 76,359	$ 194,034