Real Estate Investments - Summary of Land Parcels Sold (Details) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2017 USD ($) a Parcel	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) a Parcel	Sep. 30, 2016 USD ($)
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Proceeds from the sale of properties				$ 114,821	$ 758,931
Gain on Sale		$ 0	$ (104)	$ 8,411	$ 114,625
Land [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Parcels | Parcel		5		5
Acreage of land | a		76.4		76.4
Sales Price		$ 29,600		$ 29,600
Proceeds from the sale of properties				28,618
Gain on Sale				$ 953
50 E. Swedesford Square [Member] | Land [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Parcels | Parcel		1		1
Acreage of land | a		12.0		12.0
Sales Price		$ 7,200		$ 7,200
Proceeds from the sale of properties				7,098
Gain on Sale				$ 882
Bishops Gate [Member] | Land [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Parcels | Parcel		1		1
Acreage of land | a		49.5		49.5
Sales Price		$ 6,000		$ 6,000
Proceeds from the sale of properties				5,640
Gain on Sale	[1]			$ 71
Garza Ranch - Multi-family [Member] | Land [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Parcels | Parcel		1		1
Acreage of land | a		8.4		8.4
Sales Price		$ 11,800		$ 11,800
Proceeds from the sale of properties				11,560
Gain on Sale	[2]			$ 0
Gateway Land - Site C [Member] | Land [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Parcels | Parcel		1		1
Acreage of land | a		4.8		4.8
Sales Price		$ 1,100		$ 1,100
Proceeds from the sale of properties				1,043
Gain on Sale	[3]			$ 0
Garza Ranch - Hotel [Member] | Land [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Parcels | Parcel		1		1
Acreage of land | a		1.7		1.7
Sales Price		$ 3,500		$ 3,500
Proceeds from the sale of properties				3,277
Gain on Sale	[2]			$ 0

[1] During the fourth quarter of 2016, the Company recognized an impairment of $3.0 million. During the second quarter of 2017, the Company recognized an additional impairment of $0.3 million, representing closing costs. See “Land Impairment” section below.

[2] The Company has a continuing involvement 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 cash received at settlement was recorded as “Deferred income, gains and rent” on the Company’s consolidated balance sheet and the Company will recognize the sale once the infrastructure improvements are complete. See Note 13, “Commitments and Contingencies” for further discussion of the infrastructure improvements.

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