Real Estate Investments - Summary of Office Properties Sold (Details) $ in Thousands			3 Months Ended		9 Months Ended
		Feb. 02, 2017 property	Sep. 30, 2017 USD ($) ft²	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) ft² property	Sep. 30, 2016 USD ($)
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Net Proceeds on Sale					$ 114,821	$ 758,931
Gain (Loss) on Sale			$ 0	$ (104)	$ 8,411	$ 114,625
Philadelphia Marine Center (Marine Piers) [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Rentable Square Feet | ft²			181,900		181,900
Sales Price			$ 21,400		$ 21,400
Net Proceeds on Sale					11,200
Gain (Loss) on Sale					$ 6,500
Office Properties [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property					12
Office Properties [Member] | Two, Four A, Four B and Five Eves Drive (Evesham Corporate Center) [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property					4
Rentable Square Feet | ft²			134,794		134,794
Sales Price			$ 9,700		$ 9,700
Net Proceeds on Sale					8,650
Gain (Loss) on Sale	[1],[2]				$ (325)
Office Properties [Member] | 200, 210 & 220 Lake Drive East (Woodland Falls) [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property					3
Rentable Square Feet | ft²			215,465		215,465
Sales Price			$ 19,000		$ 19,000
Net Proceeds on Sale					17,771
Gain (Loss) on Sale	[2],[3]				$ (249)
Office Properties [Member] | 11700, 11710, 11720 & 11740 Beltsville Drive (Calverton) [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property					3
Rentable Square Feet | ft²			313,810		313,810
Sales Price			$ 9,000		$ 9,000
Net Proceeds on Sale					8,354
Gain (Loss) on Sale	[2],[4]				$ 0
Office Properties [Member] | 1200 & 1220 Concord Avenue (Concord Airport Plaza) [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property		2			2
Rentable Square Feet | ft²			350,256		350,256
Sales Price			$ 33,100		$ 33,100
Net Proceeds on Sale					32,010
Gain (Loss) on Sale	[2],[5]				$ 551
Retail Properties [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property					1
Retail Properties [Member] | 7000 Midlantic Drive [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property					1
Rentable Square Feet | ft²			10,784		10,784
Sales Price			$ 8,200		$ 8,200
Net Proceeds on Sale					7,714
Gain (Loss) on Sale	[2]				$ 1,413
Mixed Use Properties [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property					1
Mixed Use Properties [Member] | Philadelphia Marine Center (Marine Piers) [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property					1
Rentable Square Feet | ft²			181,900		181,900
Sales Price			$ 21,400		$ 21,400
Net Proceeds on Sale					11,182
Gain (Loss) on Sale	[2],[6]				$ 6,498
Office, Retail and Mixed-use Properties [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property					14
Rentable Square Feet | ft²			1,207,009		1,207,009
Sales Price			$ 100,400		$ 100,400
Net Proceeds on Sale					85,681
Gain (Loss) on Sale	[2]				$ 7,888

[1] During the first quarter of 2017, the Company recognized a $1.0 million impairment related to these properties.

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

[3] During the fourth quarter of 2016, the Company recognized a $7.3 million impairment related to these properties.

[4] During the fourth quarter of 2016, the Company recognized a $3.0 million impairment related to these properties. During the first quarter of 2017, there was a price reduction of $1.7 million under the agreement of sale and an additional impairment of $1.7 million was recognized.

[5] During the fourth quarter of 2016, the Company recognized an $11.5 million impairment related to these properties. This sale is designated as a like-kind exchange under Section 1031 of the Internal Revenue Code (“IRC”) and, as such, the proceeds, totaling $32.0 million after closing costs and prorations, were deposited with a Qualified Intermediary, as defined under the IRC. The proceeds received at closing were recorded as “Other assets” in the Company’s consolidated balance sheet. During the third quarter of 2017, the Company acquired 3000 Market Street in Philadelphia, Pennsylvania using the full balance of the Section 1031 proceeds. See “Acquisition” section above.

[6] On March 15, 2017, the Company sold its sublease interest in the Piers at Penn’s Landing (the “Marine Piers”), which includes leasehold improvements containing 181,900 net rentable square feet, and a marina, located in Philadelphia, Pennsylvania for an aggregate sales price of $21.4 million, which will be paid in two installments. On the closing date, the buyer paid $12.0 million in cash. On the second purchase price installment date, the buyer will pay $9.4 million. The second purchase price installment is due on (a) January 31, 2020, in the event that the tenant at the Marine Piers does not exercise its existing option to extend the term of the sublease or (b) January 15, 2024, in the event that the tenant does exercise its current extension option to extend the term of the sublease. In accordance with ASC 360-20, Real Estate Sales, the Company determined that it is appropriate to account for the sales transaction under the cost recovery method. The Company received cash proceeds of $11.2 million, after closing costs and prorations, and the net book value of the Marine Piers was $4.7 million, resulting in a gain on sale of $6.5 million. The remaining gain on sale of $9.4 million will be recognized on the second purchase price installment date. The Marine Piers was classified as mixed-use within the Company’s property count.