Real Estate Investments - Acquisitions (Textual) (Details) $ in Thousands					3 Months Ended		6 Months Ended
		Jun. 29, 2018 USD ($) ft² a	Mar. 22, 2018 USD ($) a	Jan. 05, 2018 USD ($) ft²	Jun. 30, 2018 USD ($) ft²	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) ft²	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Business Acquisition [Line Items]
Prepaid leasehold interests in land held for development	[1]				$ 40,177		$ 40,177		$ 0
Capitalized transaction costs							$ 7,938	$ 9,846
Area of real estate property | ft²					16,462,764		16,462,764
Net income (loss)					$ 13,006	$ 7,653	$ 57,335	$ 28,755
Seven Tower Bridge [Member]
Business Acquisition [Line Items]
Sale of ownership interest percentage				20.00%
3025 JFK Boulevard [Member]
Business Acquisition [Line Items]
Lease agreement term		99 years
Area of developed parcels of land (in acres) | a		1
Prepaid leasehold interests in land held for development		$ 15,000
Capitalized transaction costs		300
3025 JFK Boulevard [Member] | Other Assets [Member]
Business Acquisition [Line Items]
Leasehold valuation credit		5,600
3001-3003 and 3025 JFK Boulevard [Member]
Business Acquisition [Line Items]
Lease agreement term							99 years
Leasehold valuation credit applied to development		$ 2,400
Minimum floor area ratio required for realization of credit | ft²		1,200,000
Remaining leasehold valuation credit		$ 3,200
Reimbursable leasehold valuation credit		$ 3,200
3001-3003 JFK Boulevard [Member]
Business Acquisition [Line Items]
Lease agreement term			99 years
Area of developed parcels of land (in acres) | a			1
Prepaid leasehold interests in land held for development			$ 24,600
Capitalized transaction costs			300
3001-3003 JFK Boulevard [Member] | Other Assets [Member]
Business Acquisition [Line Items]
Prepaid leasehold interests in land held for development			$ 24,900
Four Tower Bridge [Member]
Business Acquisition [Line Items]
Ownership percentage				35.00%
Percentage of ownership interest after non-monetary exchange transaction				100.00%
Capitalized transaction costs				$ 100
Fair value of unencumbered acquisition related costs, to tangible and intangible assets and liabilities				$ 23,696
Service and other revenue					700		$ 1,400
Net income (loss)					$ (100)		$ 100
Four Tower Bridge [Member] | Conshohocken [Member] | Pennsylvania [Member]
Business Acquisition [Line Items]
Area of real estate property | ft²				86,021
Real estate property debt				$ 9,700

[1] As of June 30, 2018, this caption comprised leasehold interests in prepaid 99-year ground leases at 3025 and 3001-3003 JFK Boulevard, in Philadelphia, Pennsylvania. See Note 3, “Real Estate Investments,” for further information