Subsequent Events (Textual) (Details) $ in Thousands			3 Months Ended		9 Months Ended
		Oct. 13, 2016 USD ($) ft² property	Sep. 30, 2016 USD ($) ft²	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) ft² property	Sep. 30, 2015 USD ($)
Subsequent Event [Line Items]
Gain (Loss) on Sale			$ (104)	$ 6,083	$ 114,625	$ 16,673
Office Properties [Member]
Subsequent Event [Line Items]
Number of properties | property					63
Net rentable square feet | ft²			5,082,963		5,082,963
Gross sales price			$ 807,200		$ 807,200
Gain (Loss) on Sale	[1]				$ 114,601
Subsequent Event [Member] | Office Properties [Member] | 620, 640 and 660 Allendale Road [Member]
Subsequent Event [Line Items]
Number of properties | property		3
Net rentable square feet | ft²		156,669
Gross sales price		$ 12,800
Gain (Loss) on Sale		$ 2,400

[1] Gain/(Loss) on Sale is net of closing and other transaction related costs.