Segment Information - Schedule of Property Level Information by Operating Segment (Details) ft² in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015 USD ($) ft²	Jun. 30, 2014 USD ($) ft²	Jun. 30, 2015 USD ($) ft²	Jun. 30, 2014 USD ($) ft²
Segment Reporting Information [Line Items]
Square Footage (in sqft) | ft²	29,385	42,920	29,385	42,920
Rental income	$ 163,697	$ 172,407	$ 331,669	$ 344,447
Tenant reimbursements and other income	39,997	42,787	85,080	88,007
NOI	$ 114,008	$ 122,493	$ 229,192	$ 238,022
CBD Properties
Segment Reporting Information [Line Items]
Square Footage (in sqft) | ft²	15,769	21,892	15,769	21,892
Rental income	$ 108,349	$ 106,094	$ 218,192	$ 216,333
Tenant reimbursements and other income	27,683	28,465	57,380	58,706
NOI	$ 75,566	$ 71,996	$ 149,513	$ 145,166
Suburban Properties
Segment Reporting Information [Line Items]
Square Footage (in sqft) | ft²	13,616	21,028	13,616	21,028
Rental income	$ 55,348	$ 66,313	$ 113,477	$ 128,114
Tenant reimbursements and other income	12,314	14,322	27,700	29,301
NOI	$ 38,442	$ 50,497	$ 79,679	$ 92,856