Real Estate Investments - Summary of Office Properties Sold (Parenthetical) (Details)				3 Months Ended						12 Months Ended
		Mar. 31, 2017 USD ($) property	Mar. 15, 2017 USD ($) ft²	Dec. 31, 2017 USD ($) ft²	Mar. 31, 2017 USD ($) property	Dec. 31, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2018 USD ($) ft² property	Dec. 31, 2017 USD ($) ft²	Dec. 31, 2016 USD ($)	Dec. 31, 2016	Dec. 31, 2016 property	Dec. 31, 2016 ft²	Dec. 31, 2016 Office
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties | property										97
Aggregate carrying values of properties				$ 3,832,348,000						$ 3,953,319,000	$ 3,832,348,000
Provision for impairment							$ 1,800,000			$ 71,707,000	3,057,000	$ 40,517,000
Rentable Square Feet | ft²										1,414,457
Sales Price										$ 359,300,000
Proceeds from the sale of properties										324,090,000	171,860,000	784,331,000
Assets held for sale, net				$ 392,000						11,599,000	392,000
Gain/(Loss) on Sale										$ 2,932,000	$ 31,657,000	116,983,000
Office Properties [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties | property										88
Rentable Square Feet | ft²															5,239,632
Sales Price						$ 820,000,000						820,000,000
Proceeds from the sale of properties												769,799,000
Gain/(Loss) on Sale	[1]											116,983,000
200, 210 & 220 Lake Drive East (Woodland Falls) [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Carrying value of real estate prior to impairment						$ 25,800,000						25,800,000
Impairment Hold Period						10 years
Provision for impairment						$ 7,300,000
Aggregate carrying value						18,500,000						18,500,000
200, 210 & 220 Lake Drive East (Woodland Falls) [Member] | Office Properties [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Rentable Square Feet | ft²				215,465							215,465
Sales Price				$ 19,000,000							$ 19,000,000
Proceeds from the sale of properties											17,771,000
Gain/(Loss) on Sale	[1],[2]										$ (249,000)
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							181,900
Sales Price			$ 21,400,000	$ 21,400,000							$ 21,400,000
Payment by cash on purchases			12,000,000.0								12,000,000.0
Balance payment on purchases			$ 9,400,000	9,400,000							$ 9,400,000
Date of second installment payment			Jan. 31, 2020								Jan. 31, 2020
Proceeds from the sale of properties			$ 11,200,000								$ 11,200,000
Assets held for sale, net			4,700,000	4,700,000							4,700,000
Gain/(Loss) on Sale			6,500,000								6,500,000
Deferred gain on sale			$ 9,400,000	$ 9,400,000							$ 9,400,000
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					$ 1,700,000	3,000,000.0
Purchase price reduction					$ 1,700,000
11700, 11710, 11720 & 11740 Beltsville Drive (Calverton) [Member] | Office Properties [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Rentable Square Feet | ft²				313,810							313,810
Sales Price				$ 9,000,000							$ 9,000,000
Proceeds from the sale of properties											8,354,000
Gain/(Loss) on Sale	[1],[3]										0
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											$ 32,000.0
Proceeds after closing costs and prorations						11,500
1200 & 1220 Concord Avenue (Concord Airport Plaza) [Member] | Office Properties [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Rentable Square Feet | ft²				350,256							350,256
Sales Price				$ 33,100,000							$ 33,100,000
Proceeds from the sale of properties											32,010,000
Gain/(Loss) on Sale	[1]										551,000
196/198 Van Buren Street (Herndon Metro Plaza I&II) [Member] | Office Properties [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Impairment of properties held for use								$ 7,400,000
Rentable Square Feet | ft²															197,225
Sales Price						44,500,000						44,500,000
Proceeds from the sale of properties												43,412,000
Gain/(Loss) on Sale	[1],[4]											(752,000)
Och Ziff Portfolio [Member] | Office Properties [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Rentable Square Feet | ft²															3,924,783
Sales Price						398,100,000						398,100,000
Proceeds from the sale of properties												353,971,000
Gain/(Loss) on Sale	[1],[5]											(372,000)
Provision for impairment on assets held for sale									$ 45,400,000
Residual Capitalization Rates [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]
Measurement input for impairment on assets held for sale													0.0875
Discount Rates [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]
Measurement input for impairment on assets held for sale													0.0900
Other [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties		4			4					1				3		2
Impairment of properties held for use		$ 1,000,000.0								$ 14,800,000		7,300,000
Carrying value of real estate prior to impairment				10,200,000		25,800,000				$ 52,800,000	10,200,000	$ 25,800,000
Impairment Hold Period		10 years								10 years		10 years
Provision for impairment				0
Aggregate carrying value						$ 18,500,000						$ 18,500,000
Other [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 | property														3
Other [Member] | Impairment on Held for Use Properties [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Aggregate carrying values of properties				$ 9,200,000						$ 38,000,000	$ 9,200,000
Other [Member] | Residual Capitalization Rates [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Measurement input for impairment on assets held for sale		0.0900			0.0900					0.075			0.0875
Other [Member] | Discount Rates [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Measurement input for impairment on assets held for sale		0.0925			0.0925					0.095			0.0900

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

[2] As of December 31, 2016, the Company evaluated the recoverability of the carrying value of its properties that triggered assessment under the undiscounted cash flow model. Based on the Company’s evaluation, it was determined that due to the reduction in the Company’s intended hold period of three properties located in the Other segment, the Company would not recover the carrying values of these properties. Accordingly, the Company recorded impairment charges on these properties of $7.3 million at December 31, 2016, reducing the aggregate carrying values of the properties from $25.8 million to their estimated fair value of $18.5 million. The Company measured these impairments based on a discounted cash flow analysis, using a hold period of 10 years and residual capitalization rates and discount rates of 8.75% and 9.00%, respectively. The results were comparable to indicative pricing in the market. The assumptions used to determine fair value under the income approach are Level 3 inputs in accordance with the fair value hierarchy established by Accounting Standards Codification (ASC) Topic 820, “Fair Value Measurements and Disclosures.” The loss on sale in the table above represents additional closing costs.

[3] 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.

[4] During the three-month period ended March 31, 2016, the Company recognized a provision for impairment totaling approximately $7.4 million on the properties. See “Held for Use Impairment” section below. The loss on sale primarily relates to additional closing costs recognized at closing.

[5] During the three-month period ended December 31, 2015, the Company recognized a provision for impairment totaling approximately $45.4 million. The loss on sale represents additional closing costs recognized at closing.