Real Estate Investments - Summary of Land Parcels Sold (Details) $ in Thousands		12 Months Ended
		Dec. 31, 2015 USD ($) a Parcel		Dec. 31, 2014 USD ($) a Parcel		Dec. 31, 2013 USD ($)
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Net Proceeds on Sale		$ 247,228		$ 118,855		$ 423,480
Gain (Loss) on Sale		$ 20,496		$ 4,901		$ 0
Land [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Parcels | Parcel		4		2
Acreage of Land | a		14.7		22.1
Sales Price		$ 22,300		$ 5,120
Net Proceeds on Sale		21,088		4,855
Gain (Loss) on Sale		$ 3,019	[1]	$ 1,184	[2]
Two Christina Centre [Member] | Land [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Parcels | Parcel		1
Acreage of Land | a		1.6
Sales Price		$ 6,500
Net Proceeds on Sale		5,986
Gain (Loss) on Sale	[1],[3]	$ 0
A7000 Midlantic [Member] | Land [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Parcels | Parcel		1
Acreage of Land | a		3.5
Sales Price		$ 2,200
Net Proceeds on Sale		1,742
Gain (Loss) on Sale	[1]	$ (169)
Four Points Centre [Member] | Land [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Parcels | Parcel		1
Acreage of Land | a		8.6
Sales Price		$ 2,500
Net Proceeds on Sale		2,344
Gain (Loss) on Sale	[1]	$ 71
Two Kaiser Plaza [Member] | Land [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Parcels | Parcel		1
Acreage of Land | a		1.0
Sales Price		$ 11,100
Net Proceeds on Sale		11,016
Gain (Loss) on Sale	[1]	$ 3,117
West Point II Land [Member] | Land [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Parcels | Parcel				1
Acreage of Land | a				5.3
Sales Price				$ 1,600
Net Proceeds on Sale				1,505
Gain (Loss) on Sale	[2]			$ 12
Rob Roy - Land [Member] | Land [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Parcels | Parcel				1
Acreage of Land | a				16.8
Sales Price				$ 3,520
Net Proceeds on Sale				3,350
Gain (Loss) on Sale	[2]			$ 1,172

[1] Gain/(Loss) on sale includes closing and other transaction related costs.

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

[3] The Company recorded an impairment loss of $0.3 million for Two Christina Centre during the fourth quarter of 2015. As such, there was no gain/(loss) at disposition for this land parcel.