Real Estate Properties - Narrative (Details) $ in Thousands		1 Months Ended	3 Months Ended		9 Months Ended
	Jun. 27, 2014 USD ($) ft² property building	Jan. 31, 2013 property building	Sep. 30, 2015 USD ($) ft²	Sep. 30, 2014 USD ($) ft²	Sep. 30, 2015 USD ($) ft² property building	Sep. 30, 2014 USD ($) ft²	Dec. 31, 2014 property
Real Estate Properties [Line Items]
Real estate improvements					$ 46,700	$ 65,900
Number of properties held for sale | property							0
Number of properties sold or agreed to be sold | property		3
Number of buildings sold | building		18
Property square feet | ft²			25,286,000	42,919,000	25,286,000	42,919,000
Loss on asset impairment			$ 0	$ 122	$ 0	$ (2,238)
Loss on early extinguishment of debt			$ 0	$ 0	$ 0	$ (3,345)
Disposed of by Sale
Real Estate Properties [Line Items]
Number of properties sold or agreed to be sold | property					82
Number of buildings sold | building					117
Property square feet | ft²			16,368,645		16,368,645
CBD | Disposed of by Sale
Real Estate Properties [Line Items]
Number of properties sold or agreed to be sold | property	1
Number of buildings sold | building	2
Suburban property | Disposed of by Sale
Real Estate Properties [Line Items]
Number of properties sold or agreed to be sold | property	13
Number of buildings sold | building	41
CBD and suburban properties | Disposed of by Sale
Real Estate Properties [Line Items]
Property square feet | ft²	2,784,098
Proceeds from sale of real estate	$ 215,900
Loss on asset impairment	2,400
Loss on early extinguishment of debt	$ 3,300