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Summary of Significant Accounting Policies (Textual) (Details) |  |  |  |  |  | 3 Months Ended | 12 Months Ended |  |
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Dec. 21, 2018 USD ($) | Aug. 23, 2018 | May 18, 2017 shares | Mar. 15, 2017 USD ($) ft² | Jul. 01, 2016 USD ($) a | Dec. 31, 2018 USD ($) ft² a Parcel Customer shares | Jun. 30, 2018 USD ($) | Dec. 31, 2016 USD ($) Customer | Dec. 31, 2018 USD ($) ft² a Parcel Customer shares | Dec. 31, 2017 USD ($) ft² Customer | Dec. 31, 2016 USD ($) Customer | Jan. 01, 2019 USD ($) | Dec. 03, 2015 ft²
Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Income tax (provision) benefit |  |  |  |  |  |  |  |  | $ (423,000) | $ 628,000 | $ 0 |  |
Increase in operating cash flows |  |  |  |  |  |  |  |  | 14,334,000 | (1,752,000) | (544,000) |  |
Variable interest entity, assets |  |  |  |  |  | $ 414,300,000 |  |  | 414,300,000 | 412,900,000 |  |  |
Variable interest entity, liabilities |  |  |  |  |  | 254,100,000 |  |  | 254,100,000 | 250,400,000 |  |  |
Revenue increased by straight-line rent adjustment |  |  |  |  |  |  |  |  | 10,400,000 | 24,900,000 | 26,300,000 |  |
Increased revenue due to amortization of deferred rent |  |  |  |  |  |  |  |  | 1,800,000 | 2,200,000 | 2,100,000 |  |
Decrease in revenue due to amortization of lease incentives |  |  |  |  |  |  |  |  | 1,500,000 | 1,800,000 | 2,000,000.0 |  |
Lease agreement variable income |  |  |  |  |  |  |  |  | 0 |  |  |  |
Third Party Management Accounts Receivable Outstanding |  |  |  |  |  | $ 1,500,000 |  |  | $ 1,500,000 |  |  |  |
Rentable Square Feet | ft² |  |  |  |  |  | 1,414,457 |  |  | 1,414,457 |  |  |  |
Sales Price |  |  |  |  |  | $ 359,300,000 |  |  | $ 359,300,000 |  |  |  |
Proceeds from the sale of properties |  |  |  |  |  |  |  |  | 324,090,000 | 171,860,000 | 784,331,000 |  |
Assets held for sale, net |  |  |  |  |  | 11,599,000 |  |  | 11,599,000 | 392,000 |  |  |
Gain (Loss) on Sale |  |  |  |  |  |  |  |  | 2,932,000 | 31,657,000 | 116,983,000 |  |
Gross sales value of office property |  |  |  |  |  | $ 359,300,000 |  |  | 359,300,000 |  |  |  |
Proceeds from real estate venture sales |  |  |  |  |  |  |  |  | $ 60,346,000 | 145,416,000 | 21,022,000 |  |
Common shares for future awards | shares |  |  |  |  |  | 7,366,306 |  |  | 7,366,306 |  |  |  |
Additional shares authorized for issuance | shares |  |  | 2,663,886 |  |  |  |  |  |  |  |  |  |
Total number of shares authorized for issuance | shares |  |  | 6,500,000 |  |  |  |  |  |  |  |  |  |
Number of awards granted | shares |  |  |  |  |  |  |  |  | 429,434 |  |  |  |
Number of awards cancelled | shares |  |  |  |  |  |  |  |  | 1,295,740 |  |  |  |
Share-based compensation expense |  |  |  |  |  |  |  |  | $ 7,600,000 | 6,300,000 | 5,600,000 |  |
Share-based compensation expense, capitalized |  |  |  |  |  |  |  |  | $ 1,600,000 | $ 1,200,000 | $ 1,000,000.0 |  |
Employee Stock Option And Share Appreciation Rights |  |  |  |  |  |  |  |  |  |  |  |  |
Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Common shares for future awards | shares |  |  |  |  |  | 964,359 |  |  | 964,359 |  |  |  |
Garza Land Acquisition [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Area of land acquired (in acres) | a |  |  |  |  | 34.6 |  |  |  |  |  |  |  |
Aggregate gain recognized |  |  |  |  |  | $ 200,000 | $ 2,800,000 |  |  |  |  |  |
Subaru Build-to-Service Center Project [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Rentable Square Feet | ft² |  |  |  |  |  |  |  |  |  |  |  |  | 83,000
Sales Price | $ 45,300,000 |  |  |  |  |  |  |  |  |  |  |  |
Lease terms |  |  |  |  |  |  |  |  |  |  |  |  | 18 years
Purchase option of lease from inception period |  | 5 years |  |  |  |  |  |  |  |  |  |  |
Direct finance lease, interest income |  |  |  |  |  |  |  |  | $ 1,600,000 |  |  |  |
Gross sales value of office property | 45,300,000 |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from real estate venture sales | $ 44,900,000 |  |  |  |  |  |  |  |  |  |  |  |
Garza Land Sale [Member] | Garza Land Acquisition [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Area of land acquired (in acres) | a |  |  |  |  | 34.6 |  |  |  |  |  |  |  |
Gross purchase value of assets |  |  |  |  | $ 20,600,000 |  |  |  |  |  |  |  |
Number of Parcels | Parcel |  |  |  |  |  | 3 |  |  | 3 |  |  |  |
Garza Land Sale [Member] | Parcel One [Member] | Garza Land Acquisition [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Area of land sold | a |  |  |  |  |  | 8.4 |  |  | 8.4 |  |  |  |
Garza Land Sale [Member] | Parcel Two [Member] | Garza Land Acquisition [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Area of land sold | a |  |  |  |  |  | 1.7 |  |  | 1.7 |  |  |  |
Garza Land Sale [Member] | Parcel Three [Member] | Garza Land Acquisition [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Area of land sold | a |  |  |  |  |  | 6.6 |  |  | 6.6 |  |  |  |
Garza Land Sale [Member] | Third Party Developers [Member] | Parcel One [Member] | Garza Land Acquisition [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Date of parcels sold |  |  |  |  |  |  |  |  | Jan. 30, 2017 |  |  |  |
Garza Land Sale [Member] | Third Party Developers [Member] | Parcel Two [Member] | Garza Land Acquisition [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Date of parcels sold |  |  |  |  |  |  |  |  | Apr. 28, 2017 |  |  |  |
Garza Land Sale [Member] | Third Party [Member] | Parcel Three [Member] | Garza Land Acquisition [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Date of parcels sold |  |  |  |  |  |  |  |  | Mar. 16, 2018 |  |  |  |
Philadelphia Marine Center (Marine Piers) [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Rentable Square Feet | ft² |  |  |  | 181,900 |  |  |  |  |  | 181,900 |  |  |
Sales Price |  |  |  | $ 21,400,000 |  |  |  |  |  | $ 21,400,000 |  |  |
Payment by cash on purchases |  |  |  | 12,000,000.0 |  |  |  |  |  | 12,000,000.0 |  |  |
Proceeds from the sale of properties |  |  |  | 11,200,000 |  |  |  |  |  | 11,200,000 |  |  |
Balance payment on purchase price |  |  |  | $ 9,400,000 |  |  |  |  |  | $ 9,400,000 |  |  |
Date of second installment payment |  |  |  | Jan. 31, 2020 |  |  |  |  |  | Jan. 31, 2020 |  |  |
Assets held for sale, net |  |  |  | $ 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 |  |  |
Gross sales value of office property |  |  |  | $ 21,400,000 |  |  |  |  |  | $ 21,400,000 |  |  |
Customer Concentration Risk [Member] | Rental Revenue [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Number of tenant accounted for 10% or more of the Company's rents | Customer |  |  |  |  |  | 0 |  | 0 | 0 | 0 | 0 |  |
Unconsolidated Real Estate Ventures [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Capitalized interest |  |  |  |  |  |  |  |  | $ 0 | $ 400,000 | $ 1,900,000 |  |
Minimum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Buildings and improvements useful life |  |  |  |  |  |  |  |  | 5 years |  |  |  |
Tenant Improvements Useful Lives |  |  |  |  |  |  |  |  | 1 year |  |  |  |
Lease terms |  |  |  |  |  | 10 years |  |  | 10 years |  |  |  |
Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Buildings and improvements useful life |  |  |  |  |  |  |  |  | 55 years |  |  |  |
Tenant Improvements Useful Lives |  |  |  |  |  |  |  |  | 16 years |  |  |  |
Lease terms |  |  |  |  |  | 66 years |  |  | 66 years |  |  |  |
ASU 2016-18 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted cash |  |  |  |  |  | $ 400,000 |  | $ 700,000 | $ 400,000 | 1,300,000 | 700,000 |  |
Increase in operating cash flows |  |  |  |  |  |  |  |  |  | 1,300,000 | $ 700,000 |  |
ASU No. 2016-02, Leases (Topic 842) [Member] | Scenario Plan [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Lease liabilities |  |  |  |  |  |  |  |  |  |  |  | $ 22,400,000 |
Right of use assets |  |  |  |  |  |  |  |  |  |  |  | $ 22,400,000 |
Impairment Charges [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Out-of-period error immaterial in current year |  |  |  |  |  |  |  | $ 1,200,000 |  |  |  |  |
General And Administrative Expense [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Income tax (provision) benefit |  |  |  |  |  |  |  |  |  | 600,000 |  |  |
Income Tax Expense Benefit [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Reclassification of net income tax provision out of general and administrative expenses into net income tax provision |  |  |  |  |  |  |  |  |  | $ 600,000 |  |  |

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