Real Estate Investments - Summary of Land Parcels Sold (Details) $ in Thousands		12 Months Ended
		Dec. 31, 2017 USD ($) a Parcel	Dec. 31, 2016 USD ($) a Parcel		Dec. 31, 2015 USD ($) a Parcel		Jan. 15, 2016 Parcel
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Proceeds from the sale of properties		$ 171,860	$ 784,331		$ 247,228
Gain (Loss) on Sale		$ 953	$ 9,232		$ 3,019
Greenhills Land [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Parcels | Parcel							120
Land [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Parcels | Parcel		5	3		4
Acreage of land | a		76.4	122.9		14.7
Sales Price		$ 29,600	$ 14,938		$ 22,300
Proceeds from the sale of properties		28,618	14,532		21,088
Gain (Loss) on Sale		$ 953	$ 9,232	[1]	$ 3,019	[2]
Land [Member] | 50 E. Swedesford Square [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		12.0
Sales Price		$ 7,200
Proceeds from the sale of properties		7,098
Gain (Loss) on Sale		$ 882
Land [Member] | Bishops Gate [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		49.5
Sales Price		$ 6,000
Proceeds from the sale of properties		5,640
Gain (Loss) on Sale	[3]	$ 71
Land [Member] | Garza Ranch - Multi-family [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		8.4
Sales Price		$ 11,800
Proceeds from the sale of properties		11,560
Gain (Loss) on Sale	[4]	$ 0
Land [Member] | Gateway Land - Site C [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		4.8
Sales Price		$ 1,100
Proceeds from the sale of properties		1,043
Gain (Loss) on Sale	[5]	$ 0
Land [Member] | Garza Ranch - Hotel [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		1.7
Sales Price		$ 3,500
Proceeds from the sale of properties		3,277
Gain (Loss) on Sale	[4]	$ 0
Land [Member] | Oakland Lot B [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			0.9
Sales Price			$ 13,750
Proceeds from the sale of properties			13,411
Gain (Loss) on Sale	[1]		$ 9,039
Land [Member] | Highlands 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			2.0
Sales Price			$ 288
Proceeds from the sale of properties			284
Gain (Loss) on Sale	[1]		$ 193
Land [Member] | Greenhills 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			120.0
Sales Price			$ 900
Proceeds from the sale of properties			837
Gain (Loss) on Sale	[1],[6]		$ 0
Land [Member] | Two Christina Centre [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
Proceeds from the sale of properties					5,986
Gain (Loss) on Sale	[2],[7]				$ 0
Land [Member] | A7000 Midlantic [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
Proceeds from the sale of properties					1,742
Gain (Loss) on Sale	[2]				$ (169)
Land [Member] | Four Points Centre [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
Proceeds from the sale of properties					2,344
Gain (Loss) on Sale	[2]				$ 71
Land [Member] | Two Kaiser Plaza [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
Proceeds from the sale of properties					11,016
Gain (Loss) on Sale	[2]				$ 3,117

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

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

[3] During the fourth quarter of 2016, the Company recognized an impairment of $3.0 million. During the second quarter of 2017, the Company recorded a held for sale impairment charge of $0.3 million, reducing the aggregate carrying value of the land parcel from $5.9 million to its estimated fair value less costs to sell of $5.6 million. The fair value measurement is based on pricing in the purchase and sale agreement for the property. As the pricing in the purchase and sale agreement is unobservable, the Company determined that the input utilized to determine fair value for the property falls within Level 3 in accordance with the fair value hierarchy established by Accounting Standards Codification (ASC) Topic 820, “Fair Value Measurements and Disclosures.” The land parcel was sold on July 18, 2017.

[4] The Company has a continuing involvement in this property through a completion guaranty, which requires the Company, as developer, to complete certain infrastructure improvements on behalf of the buyers of the land parcels. The Company recorded the cash received at settlement as “Deferred income, gains and rent” on the Company’s consolidated balance sheet and the Company will recognize the sale upon completion of infrastructure improvements. See Note 19, “Commitments and Contingencies” for further discussion of the infrastructure improvements.

[5] During the fourth quarter of 2016, the Company recognized a nominal impairment related to this land parcel.

[6] The carrying value of the land exceeded the fair value less the anticipated costs of sale as of December 31, 2015. Therefore the Company recognized an impairment loss of $0.3 million during the three-month period ended December 31, 2015. There was no gain or loss recognized on the sale during 2016.

[7] 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.