Commitments And Contingencies (Narrative) (Details) $ in Thousands	Aug. 02, 2017 USD ($) ft² property contract	Jun. 30, 2017 USD ($) ft² property	Dec. 31, 2016 USD ($)
Other Commitments [Line Items]
Number of properties | property		178
Area of real estate property | ft²		12,700,000
Net book value | $		$ 1,345,606	$ 1,233,681
Outstanding Contract With Third Party [Member]
Other Commitments [Line Items]
Outstanding contracts with a third-party | contract	4
Washington D.C. [Member]
Other Commitments [Line Items]
Number of properties | property		28
Area of real estate property | ft²		2,800,000
Washington D.C. [Member] | Outstanding Contract With Third Party [Member]
Other Commitments [Line Items]
Outstanding contracts with a third-party | contract	2
Number of properties | property	3
Area of real estate property | ft²	583,000
Total aggregate initial investment | $	$ 52,000
Net book value | $	$ 29,300
Industrial Building [Member] | Non-binding Letter Of Intent [Member]
Other Commitments [Line Items]
Number of non-binding letters of intent | contract	1
Number of properties | property	1
Area of real estate property | ft²	38,000
Total aggregate initial investment | $	$ 7,100
Industrial Building [Member] | Outstanding Contract With Third Party [Member]
Other Commitments [Line Items]
Number of properties | property	4
Area of real estate property | ft²	424,000