Real Estate Properties - Narrative (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) ft² building	Dec. 31, 2018 USD ($) building	Dec. 31, 2017 USD ($) ft² building property
Real Estate Properties [Line Items]
Real estate property improvements	$ 13,900	$ 56,300	$ 55,000
Committed expenditures on leases executed during period	$ 25,500
Square feet of leases committed for expenditures during the period (in sqft) | ft²	600,000
Committed but unspent tenant related obligations based on existing leases	$ 73,900
Disposed of by Sale
Real Estate Properties [Line Items]
Number of properties sold | property			16
Number of buildings sold | building	6	9	37
Property square feet (in sqft) | ft²			6,588,128
Gross sales price	$ 812,100	$ 1,031,286	$ 862,600