Real Estate Investments - Summary of Office Properties Sold (Parenthetical) (Details) $ in Thousands	3 Months Ended				9 Months Ended
	Sep. 30, 2017 USD ($) ft²	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) ft² Installment	Sep. 30, 2016 USD ($)
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
Assets held for sale, net	$ 31,125		$ 41,718		31,125
Net gain (loss) on disposition of real estate	$ 0			$ (104)	$ 8,411	$ 114,625
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]
Provision for impairment on assets held for sale		$ 1,000
200, 210 & 220 Lake Drive East (Woodland Falls) [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Provision for impairment on assets held for sale			7,300
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
Aggregate sales price	$ 21,400				$ 21,400
Number of installments | Installment					2
First installment payment					$ 12,000
Second installment payment	9,400				$ 9,400
Date of second installment payment					Jan. 31, 2020
Proceeds from the sale of properties					$ 11,200
Assets held for sale, net		4,700
Net gain (loss) on disposition of real estate					6,500
Deferred gain on sale	$ 9,400				9,400
11700, 11710, 11720 & 11740 Beltsville Drive (Calverton) [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Provision for impairment on assets held for sale		1,700	3,000
Purchase price reduction		$ 1,700
1200 & 1220 Concord Avenue (Concord Airport Plaza) [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Provision for impairment on assets held for sale			$ 11,500
Proceeds after closing costs and prorations					$ 32,000