Segments - Net Income of Reportable Segments (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
Rental revenues	$ 27,654	$ 27,096	$ 26,755	$ 27,232	$ 26,516	$ 25,305	$ 24,251	$ 23,283	$ 108,737	$ 99,355	$ 81,172
Rental expenses									25,422	21,904	19,204
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
General contracting and real estate services expenses									186,590	153,375	165,344
Real estate taxes									10,528	9,629	7,782
Net operating income	$ 19,211	$ 19,397	$ 20,645	$ 20,978	$ 19,740	$ 18,393	$ 17,973	$ 17,371
Operating Segments
Segment Reporting Information
Net operating income									80,231	73,477	60,110
Operating Segments | Office real estate
Segment Reporting Information
Rental revenues									19,207	20,929	31,534
Rental expenses									5,483	5,560	6,938
Real estate taxes									1,859	2,000	2,950
Net operating income									11,865	13,369	21,646
Operating Segments | Retail real estate
Segment Reporting Information
Rental revenues									63,109	56,511	32,064
Rental expenses									10,233	9,116	5,915
Real estate taxes									6,176	5,395	2,928
Net operating income									46,700	42,000	23,221
Operating Segments | Multifamily residential real estate
Segment Reporting Information
Rental revenues									26,421	21,915	17,574
Rental expenses									9,705	7,228	6,351
Real estate taxes									2,494	2,234	1,904
Net operating income									14,222	12,453	9,319
Operating Segments | General contracting and real estate services
Segment Reporting Information
General contracting and real estate services revenues									194,034	159,030	171,268
General contracting and real estate services expenses									186,590	153,375	165,344
Net operating income									$ 7,444	$ 5,655	$ 5,924