Segment Information - Summary of NOI by Segment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Property rentals	$ 436,625	$ 401,595	$ 389,810
Tenant reimbursements	37,985	37,661	40,476
Total rental revenue	474,610	439,256	430,286
Property operating	111,055	100,304	101,511
Real estate taxes	66,434	57,784	58,874
Total rental expense	177,489	158,088	160,385
Consolidated NOI	297,121	281,168	269,901
Operating Segments | Office
Segment Reporting Information [Line Items]
Property rentals	343,213	317,956	311,671
Tenant reimbursements	32,315	33,361	35,508
Total rental revenue	375,528	351,317	347,179
Property operating	93,834	91,128	92,355
Real estate taxes	50,483	46,115	45,479
Total rental expense	144,317	137,243	137,834
Consolidated NOI	231,211	214,074	209,345
Operating Segments | Multifamily
Segment Reporting Information [Line Items]
Property rentals	85,809	63,401	53,071
Tenant reimbursements	5,012	3,454	2,790
Total rental revenue	90,821	66,855	55,861
Property operating	24,297	17,238	14,606
Real estate taxes	10,940	6,993	6,022
Total rental expense	35,237	24,231	20,628
Consolidated NOI	55,584	42,624	35,233
Operating Segments | Other
Segment Reporting Information [Line Items]
Property rentals	10,508	23,234	27,504
Tenant reimbursements	658	846	2,178
Total rental revenue	11,166	24,080	29,682
Property operating	8,528	7,216	9,268
Real estate taxes	5,011	4,676	7,373
Total rental expense	13,539	11,892	16,641
Consolidated NOI	(2,373)	12,188	13,041
Elimination of Intersegment Activity
Segment Reporting Information [Line Items]
Property rentals	(2,905)	(2,996)	(2,436)
Tenant reimbursements	0	0	0
Total rental revenue	(2,905)	(2,996)	(2,436)
Property operating	(15,604)	(15,278)	(14,718)
Real estate taxes	0	0	0
Total rental expense	(15,604)	(15,278)	(14,718)
Consolidated NOI	$ 12,699	$ 12,282	$ 12,282