Real Estate Properties - Narrative (Details) $ in Thousands	1 Months Ended		12 Months Ended
	Oct. 31, 2016 USD ($)	Jan. 31, 2013 building property	Dec. 31, 2017 USD ($) ft² building	Dec. 31, 2016 USD ($) building property	Dec. 31, 2015 USD ($) ft² building property
Real Estate Properties [Line Items]
Real estate property improvements			$ 55,000	$ 110,700	$ 75,700
Committed expenditures on leases executed during period			$ 44,900
Square feet of leases committed for expenditures during the period (in sqft) | ft²			1,300,000
Committed but unspent tenant related obligations based on existing leases			$ 63,200
Number of real estate properties held for sale | property				0
Number of properties sold | property		3
Number of buildings sold | building		18
Ground lease expenses				$ 900	$ 1,800
Disposed of by Sale
Real Estate Properties [Line Items]
Number of properties sold | property					91
Number of buildings sold | building			37	62	135
Property square feet (in sqft) | ft²					18,939,185
Gross sales price			$ 862,624	$ 1,259,895	$ 2,000,000
Austin, Texas
Real Estate Properties [Line Items]
Payments to acquire land	$ 2,800