Segment Information - Summary of NOI by Segment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Segment Reporting Information [Line Items]
Property rentals	$ 116,458	$ 103,265	$ 316,899	$ 299,497
Tenant reimbursements	9,593	10,231	27,161	28,428
Total rental revenue	126,051	113,496	344,060	327,925
Property operating	29,634	27,287	77,341	75,087
Real estate taxes	17,194	14,462	47,978	43,712
Total rental expense	46,828	41,749	125,319	118,799
NOI	79,223	71,747	218,741	209,126
Operating Segments | Office
Segment Reporting Information [Line Items]
Property rentals	91,534	81,575	249,532	237,826
Tenant reimbursements	7,917	8,977	22,738	24,807
Total rental revenue	99,451	90,552	272,270	262,633
Property operating	27,000	25,083	71,377	69,740
Real estate taxes	13,038	11,793	37,185	34,855
Total rental expense	40,038	36,876	108,562	104,595
NOI	59,413	53,676	163,708	158,038
Operating Segments | Multifamily
Segment Reporting Information [Line Items]
Property rentals	23,397	15,850	62,050	45,203
Tenant reimbursements	1,548	876	3,772	2,422
Total rental revenue	24,945	16,726	65,822	47,625
Property operating	6,796	4,782	16,716	12,594
Real estate taxes	2,952	1,663	7,973	5,063
Total rental expense	9,748	6,445	24,689	17,657
NOI	15,197	10,281	41,133	29,968
Operating Segments | Other
Segment Reporting Information [Line Items]
Property rentals	4,171	4,898	9,623	18,621
Tenant reimbursements	128	378	651	1,199
Total rental revenue	4,299	5,276	10,274	19,820
Property operating	3,502	3,065	11,330	14,934
Real estate taxes	1,204	1,006	2,820	3,794
Total rental expense	4,706	4,071	14,150	18,728
NOI	(407)	1,205	(3,876)	1,092
Eliminations
Segment Reporting Information [Line Items]
Property rentals	(2,644)	942	(4,306)	(2,153)
Tenant reimbursements	0	0	0	0
Total rental revenue	(2,644)	942	(4,306)	(2,153)
Property operating	(7,664)	(5,643)	(22,082)	(22,181)
Real estate taxes	0		0	0
Total rental expense	(7,664)	(5,643)	(22,082)	(22,181)
NOI	$ 5,020	$ 6,585	$ 17,776	$ 20,028