Selected Quarterly Financial Data (Unaudited) - Summary Certain Selected Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended								12 Months Ended
	Jan. 08, 2016	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
Quarterly Financial Data [Abstract]
Rental revenues		$ 27,654	$ 27,096	$ 26,755	$ 27,232	$ 26,516	$ 25,305	$ 24,251	$ 23,283	$ 108,737	$ 99,355	$ 81,172
General contracting and real estate services revenues		32,643	41,201	56,671	63,519	50,475	38,552	33,200	36,803	194,034	159,030	171,268
Net operating income		19,211	19,397	20,645	20,978	19,740	18,393	17,973	17,371
Net income		5,768	10,461	4,943	8,753	5,145	7,946	3,131	26,533	$ 29,925	$ 42,755	$ 31,183
Net income attributable to stockholders		$ 4,152	$ 7,488	$ 3,471	$ 5,936	$ 3,458	$ 5,212	$ 2,034	$ 17,370
Net income per share: basic and diluted (in dollars per share)		$ 0.09	$ 0.17	$ 0.08	$ 0.16	$ 0.09	$ 0.15	$ 0.06	$ 0.57	$ 0.50	$ 0.85	$ 0.75
Disposal Group, Disposed of by Sale, Not Discontinued Operations | Richmond Tower
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (loss) on disposition of property	$ 26,200