Real Estate Investments - Summary of Office Properties Sold (Details) $ in Thousands		3 Months Ended
		Mar. 31, 2017 USD ($) ft² property	Mar. 31, 2016 USD ($)
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Net Proceeds on Sale		$ 74,159	$ 704,911
Gain (Loss) on Sale		$ 7,323	$ 115,456
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
Sales Price		$ 21,400
Net Proceeds on Sale		11,200
Gain (Loss) on Sale		$ 6,500
Office Building and Mixed-use Property [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property		9
Rentable Square Feet | ft²		1,061,431
Sales Price		$ 82,500
Net Proceeds on Sale		69,317
Gain (Loss) on Sale	[1]	$ 6,800
Office Building and Mixed-use Property [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
Sales Price		$ 19,000
Net Proceeds on Sale		17,771
Gain (Loss) on Sale	[1],[2]	$ (249)
Office Building and Mixed-use Property [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
Sales Price		$ 21,400
Net Proceeds on Sale		11,182
Gain (Loss) on Sale	[1],[3]	$ 6,498
Office Building and Mixed-use Property [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
Sales Price		$ 9,000
Net Proceeds on Sale		8,354
Gain (Loss) on Sale	[1],[4]	$ 0
Office Building and Mixed-use Property [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
Rentable Square Feet | ft²		350,256
Sales Price		$ 33,100
Net Proceeds on Sale		32,010
Gain (Loss) on Sale	[1],[5]	$ 551

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

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

[3] 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 a 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.

[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] 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 as of March 31, 2017. During the fourth quarter of 2016, the Company recognized a $11.5 million impairment related to these properties.