Segment Information - Schedule of Property Level Information by Operating Segment (Details) ft² in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015 USD ($) ft²	Sep. 30, 2014 USD ($) ft²	Sep. 30, 2015 USD ($) ft²	Sep. 30, 2014 USD ($) ft²
Segment Reporting Information [Line Items]
Square Footage (in sqft) | ft²	25,286	42,919	25,286	42,919
Rental income	$ 125,459	$ 174,216	$ 457,128	$ 518,663
Tenant reimbursements and other income	33,749	42,379	118,829	130,386
NOI	$ 85,637	$ 117,203	$ 314,829	$ 355,225
Operating Segments | CBD Properties
Segment Reporting Information [Line Items]
Square Footage (in sqft) | ft²	13,975	21,892	13,975	21,892
Rental income	$ 85,494	$ 109,822	$ 303,686	$ 326,155
Tenant reimbursements and other income	23,054	27,653	80,434	86,359
NOI	$ 53,564	$ 71,596	$ 203,077	$ 216,762
Operating Segments | Suburban Properties
Segment Reporting Information [Line Items]
Square Footage (in sqft) | ft²	11,311	21,027	11,311	21,027
Rental income	$ 39,965	$ 64,394	$ 153,442	$ 192,508
Tenant reimbursements and other income	10,695	14,726	38,395	44,027
NOI	$ 32,073	$ 45,607	$ 111,752	$ 138,463