Real Estate Properties - Narrative (Details) $ in Thousands	1 Months Ended			12 Months Ended
	Oct. 31, 2016 USD ($)	Mar. 31, 2014 USD ($) ft² building property	Jan. 31, 2013 building property	Dec. 31, 2016 USD ($) ft² building property	Dec. 31, 2015 USD ($) building property	Dec. 31, 2014 USD ($) ft² building property
Real Estate Properties [Line Items]
Real estate property improvements				$ 110,700	$ 75,700	$ 91,600
Committed expenditures on leases executed during period				$ 143,000
Square feet of leases committed for expenditures during the period | ft²				4,300,000
Committed but unspent tenant related obligations based on existing leases				$ 91,800
Number of real estate properties held for use | property		31
Number of buildings held for use | building		67
Property square feet | ft²		5,641,450
Reversal of previously recorded impairment losses		$ 4,800
Number of real estate properties held for sale | property				0	0
Number of properties sold or agreed to be sold | property			3
Number of buildings sold | building			18
Loss (gain) on asset impairment from discontinued operations				$ 0	$ 0	2,238
Gain (loss) on early extinguishment of debt from discontinued operations				0	0	3,345
Ground lease expenses				$ 900	$ 1,800	$ 1,800
Disposed of by Sale
Real Estate Properties [Line Items]
Property square feet | ft²						2,784,098
Number of properties sold or agreed to be sold | property						14
Number of buildings sold | building				62	135	43
Gross Sales Price				$ 1,259,895	$ 1,999,413	$ 215,900
Loss (gain) on asset impairment from discontinued operations						2,200
Gain (loss) on early extinguishment of debt from discontinued operations						$ (3,300)
Austin, Texas
Real Estate Properties [Line Items]
Payments to acquire land	$ 2,800