Segment Information - Summary of Certain Balance Sheet Data by Segment (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Assets Held-for-sale, Not Part of Disposal Group	$ 137,455	$ 8,293
Real estate, at cost	5,693,545	6,017,504
Investments in and advances to unconsolidated real estate ventures	361,014	261,811
Total assets (1)	6,011,194	6,071,807
Operating Segments | Office
Segment Reporting Information [Line Items]
Assets Held-for-sale, Not Part of Disposal Group	130,800	1,700
Real estate, at cost	3,420,068	3,953,314
Investments in and advances to unconsolidated real estate ventures	211,301	124,659
Total assets (1)	3,488,847	3,542,977
Operating Segments | Multifamily
Segment Reporting Information [Line Items]
Assets Held-for-sale, Not Part of Disposal Group	2,200
Real estate, at cost	1,599,912	1,476,423
Investments in and advances to unconsolidated real estate ventures	105,028	98,835
Total assets (1)	1,475,233	1,434,999
Operating Segments | Other
Segment Reporting Information [Line Items]
Assets Held-for-sale, Not Part of Disposal Group	4,500	6,600
Real estate, at cost	673,565	587,767
Investments in and advances to unconsolidated real estate ventures	44,685	38,317
Total assets (1)	1,211,537	1,299,085
Elimination of Intersegment Activity
Segment Reporting Information [Line Items]
Real estate, at cost	0	0
Investments in and advances to unconsolidated real estate ventures	0	0
Total assets (1)	$ (164,423)	$ (205,254)