Real Estate Investments - Acquisitions (Textual) (Details) $ in Thousands										3 Months Ended									5 Months Ended	12 Months Ended
		Dec. 19, 2018 USD ($) ft² property	Dec. 11, 2018 USD ($) ft² property	Jun. 29, 2018 USD ($) ft²	Mar. 22, 2018 USD ($)	Jan. 05, 2018 USD ($) ft²	Oct. 13, 2017 USD ($) ft²	Jul. 28, 2017 USD ($) ft²	Jul. 01, 2016 USD ($) a	Dec. 31, 2018 USD ($) ft² property	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) a	Dec. 31, 2017 USD ($) a	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) a	Dec. 31, 2018 USD ($) ft² property	Dec. 31, 2017 USD ($) a	Dec. 31, 2016 USD ($) ft²	Nov. 01, 2018 USD ($)	Jan. 30, 2017 a
Business Acquisition [Line Items]
Number of Properties acquired | property										97										97
Rentable Square Feet | ft²										1,414,457										1,414,457
Proceeds from credit facility borrowings																				$ 455,500	$ 341,000	$ 195,000
Area of Real Estate Property | ft²										16,777,249										16,777,249
Net gain from remeasurement of investment in real estate ventures																				$ 142,233	80,526	20,000
Gain on promoted interest in unconsolidated real estate venture																				28,283	0	0
Prepaid leasehold interests in land held for development, net	[1]									$ 39,999				$ 0	$ 0				$ 0	39,999	0	0
Capitalized transaction costs																				18,407	17,657	16,083
Net income (loss)										$ 121,823	$ (43,003)	$ 12,920	$ 44,215	$ 73,137		$ 18,803	$ 4,092	$ 19,278		136,324	120,850	40,191
Amount funded by corporate funds																				$ 196,625	$ 72,523	$ 20,406
Garza Land Sale [Member]
Business Acquisition [Line Items]
Area of land under agreement to sell | a																								1.7
Office Properties [Member]
Business Acquisition [Line Items]
Number of Properties acquired | property										88										88
Rentable Square Feet | ft²																						5,239,632
Area of Real Estate Property | ft²										15,609,156										15,609,156
Seven Tower Bridge [Member]
Business Acquisition [Line Items]
Ownership percentage	[2],[3]									20.00%										20.00%
Sale of ownership interest percentage						20.00%
Quarry Lake II [Member]
Business Acquisition [Line Items]
Number of Properties acquired | property		1
Rentable Square Feet | ft²		120,559
Gross purchase value of office property		$ 39,500
Capitalized transaction costs		100
Proceeds from credit facility borrowings		39,000
Mortgage debt assumed in connection with acquisition of real estate properties		0
Working capital obtained		368
Aggregate cash payment		39,037
Mortgage debt assumed repaid at settlement		$ 0
Service and other revenue																				$ 100
Net income (loss)																				100
DRA Austin Venture [Member]
Business Acquisition [Line Items]
Number of Properties acquired | property			12
Gross purchase value of office property			$ 535,100
Ownership percentage			50.00%
Area of Real Estate Property | ft²			1,570,123
Mortgage debt assumed in connection with acquisition of real estate properties			$ 0																				$ 115,500
Investment in real estate venture			14,600
Credit settlement for notes receivable provided to venture																							$ 130,700
Working capital obtained			24,865
Cash proceeds for promoted interest in venture			28,300
Net gain from remeasurement of investment in real estate ventures			103,800							$ 103,800
Aggregate cash payment			117,348
Mortgage debt assumed repaid at settlement			115,461
Gain on promoted interest in unconsolidated real estate venture			$ 28,300							$ 28,300
Percentage of ownership interests			50.00%
Service and other revenue																				3,400
Net income (loss)																				$ (1,300)
3025 JFK Boulevard [Member]
Business Acquisition [Line Items]
Lease agreement term				99 years
Prepaid leasehold interests in land held for development, net				$ 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
Prepaid leasehold interests in land held for development, net					$ 24,600
Capitalized transaction costs					$ 300
Four Tower Bridge [Member]
Business Acquisition [Line Items]
Capitalized transaction costs						$ 100
Ownership percentage						35.00%
Mortgage debt assumed in connection with acquisition of real estate properties						$ 9,940
Working capital obtained						(1,379)
Aggregate cash payment						0
Mortgage debt assumed repaid at settlement						$ 0
Percentage of ownership interest after non-monetary exchange transaction						100.00%
Fair value of unencumbered acquisition related costs, to tangible and intangible assets and liabilities						$ 23,600
Service and other revenue																				$ 2,800
Net income (loss)																				$ 300
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
Bulletin Building [Member]
Business Acquisition [Line Items]
Rentable Square Feet | ft²							282,709
Gross purchase value of office property							$ 35,000
Capitalized transaction costs							$ 2,800
Lease agreement term							99 years
Service and other revenue															1,200
Net income (loss)															$ (400)
3000 Market Street [Member]
Business Acquisition [Line Items]
Gross purchase value of office property								$ 32,700
Capitalized transaction costs								700
Service and other revenue																			800
Net income (loss)																			$ (500)
Contractual sale price of real estate acquired								$ 32,000
3000 Market Street [Member] | Office Properties [Member]
Business Acquisition [Line Items]
Area of Real Estate Property | ft²								58,587
Garza Land Acquisition [Member]
Business Acquisition [Line Items]
Area of land acquired (in acres) | a									34.6
Gross purchase value of assets									$ 20,600
Acquisition costs									1,900
Amount funded by corporate funds									$ 20,400
Area of land under agreement to sell | a														9.5	9.5				9.5		9.5
Garza Land Acquisition [Member] | Garza Land Sale [Member]
Business Acquisition [Line Items]
Area of land acquired (in acres) | a									34.6

[1] As of December 31, 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.

[2] On January 5, 2018, the Company exchanged its 20% interest in Seven Tower Bridge to acquire the remaining 35% interest in Four Tower Bridge. For further information regarding the accounting of the transaction, see “Four Tower Bridge Acquisition” section below.

[3] Ownership percentage represents the Company’s entitlement to residual distributions after payments of priority returns, where applicable.