Real Estate Investments - Summary of Office Properties Sold (Parenthetical) (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2017 USD ($) ft² Installment	Dec. 31, 2016 USD ($)	Mar. 31, 2016 USD ($)
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Proceeds from the sale of properties	$ 74,159		$ 704,911
Assets held for sale, net	6,262	$ 41,718
Net gain on disposition of real estate	$ 7,323		$ 115,456
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
Sales Price	$ 21,400
Number of installments | Installment	2
First installment payment	$ 12,000
Second installment payment	$ 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 on disposition of real estate	6,500
Deferred gain on sale	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