Significant Acquisitions and Dispositions (Summary of Significant Assumptions Utilized in Estimates) (Details) - $ / sqft	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Minimum [Member]
Fair Value Inputs, Cap Rate	4.03%	6.25%
Maximum [Member]
Fair Value Inputs, Cap Rate	5.65%	6.96%
Duke/Quantico LLC [Member] | Minimum [Member]
Fair Value Inputs, Cap Rate		6.50%
Duke/Quantico LLC [Member] | Maximum [Member]
Fair Value Inputs, Cap Rate		9.00%
Office [Member] | Duke/Quantico LLC [Member] | Minimum [Member]
Fair Value Measurement With Unobservable Inputs Rental Rate Per Square Foot		9.34
Office [Member] | Duke/Quantico LLC [Member] | Maximum [Member]
Fair Value Measurement With Unobservable Inputs Rental Rate Per Square Foot		18.54
Industrial [Member] | Minimum [Member]
Fair Value Measurement With Unobservable Inputs Rental Rate Per Square Foot	3.50	3.35
Industrial [Member] | Maximum [Member]
Fair Value Measurement With Unobservable Inputs Rental Rate Per Square Foot	10.00	3.39
Industrial [Member] | Duke/Quantico LLC [Member] | Minimum [Member]
Fair Value Measurement With Unobservable Inputs Rental Rate Per Square Foot		8.20
Industrial [Member] | Duke/Quantico LLC [Member] | Maximum [Member]
Fair Value Measurement With Unobservable Inputs Rental Rate Per Square Foot		8.50
Medical Office [Member] | Minimum [Member]
Fair Value Measurement With Unobservable Inputs Rental Rate Per Square Foot		15.40
Medical Office [Member] | Maximum [Member]
Fair Value Measurement With Unobservable Inputs Rental Rate Per Square Foot		15.40