Real Estate Properties - FPO Transaction (Details) $ in Thousands		3 Months Ended
	Oct. 02, 2017 USD ($) ft² building joint_venture property	Mar. 31, 2018 building joint_venture property
Business Acquisition [Line Items]
Number of joint ventures | joint_venture		2
Number of properties owned | property		107
Number of buildings | building		166
First Potomac Realty Trust
Business Acquisition [Line Items]
Number of properties acquired | property	35
Number of buildings acquired | building	72
Rentable area of properties (in square feet) | ft²	6,028,072
Purchase price | $	$ 1,370,888
First Potomac Realty Trust
Business Acquisition [Line Items]
Number of joint ventures | joint_venture	2
Number of properties owned | property	2
Number of buildings | building	3
First Potomac Realty Trust | First Potomac Realty Trust
Business Acquisition [Line Items]
Rentable area of properties (in square feet) | ft²	443,867
Joint Venture Property 1 | First Potomac Realty Trust
Business Acquisition [Line Items]
Percentage of ownership interest	50.00%
Joint Venture Property 2 | First Potomac Realty Trust
Business Acquisition [Line Items]
Percentage of ownership interest	51.00%