Properties Held for Sale and Property Dispositions (Properties Held for Sale) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Depreciable real estate reserves	$ (27,795)	$ (9,955)	$ (81,605)	$ (9,973)
125 Chubb Avenue
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Depreciable real estate reserves	26,600
680/750 Washington Boulevard
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Depreciable real estate reserves	$ 2,100