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Real Estate Investments - Acquisitions (Textual) (Details) $ in Thousands |  |  |  |  |  |  |  |  | 3 Months Ended | 5 Months Ended | 12 Months Ended |  |
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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.

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