Real Estate Properties - Narrative (Details) $ in Thousands	1 Months Ended	12 Months Ended
	Oct. 31, 2016 USD ($)	Dec. 31, 2018 USD ($) ft² building	Dec. 31, 2017 USD ($) building	Dec. 31, 2016 USD ($) ft² building property
Real Estate Properties [Line Items]
Real estate property improvements		$ 56,300	$ 55,000	$ 110,700
Committed expenditures on leases executed during period		$ 98,700
Square feet of leases committed for expenditures during the period (in sqft) | ft²		1,100,000
Committed but unspent tenant related obligations based on existing leases		$ 86,600
Ground lease expenses				$ 900
Disposed of by Sale
Real Estate Properties [Line Items]
Number of properties sold | property				30
Number of buildings sold | building		9	37	62
Property square feet (in sqft) | ft²				7,972,692
Gross sales price		$ 1,031,286	$ 862,624	$ 1,300,000
Austin, Texas
Real Estate Properties [Line Items]
Payments to acquire land	$ 2,800