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Real Estate Investments - Acquisitions (Textual) (Details) $ in Thousands |  |  |  |  |  |  |  |  | 12 Months Ended |  |  |
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Jul. 07, 2015 USD ($) a | Jun. 22, 2015 USD ($) ft² a property | May. 12, 2015 USD ($) ft² | Apr. 06, 2015 USD ($) a | Apr. 02, 2015 USD ($) ft² | Feb. 19, 2014 USD ($) ft² a property Floor | Dec. 19, 2013 USD ($) ft² a Storey | Apr. 25, 2013 USD ($) a | Dec. 31, 2015 USD ($) ft² Storey | Dec. 31, 2014 USD ($) | Dec. 31, 2013 USD ($) | Dec. 29, 2015 USD ($) | Jun. 30, 2014 USD ($) | Nov. 19, 2013 USD ($) a
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rentable Square Feet | ft² |  |  |  |  |  |  |  |  |  | 4,300,000 |  |  |  |  |
Ownership percentage | [1] |  |  |  |  |  |  |  |  | 0.00% |  |  |  |  |
Net gain from remeasurement of investments in real estate ventures |  |  |  |  |  |  |  |  |  | $ 758 | $ 458 | $ 6,866 |  |  |
Development in Progress |  |  |  |  |  |  |  |  |  | 268,983 | 201,360 |  |  |  |
Change in operating real estate due to non-cash adjustment to land |  |  |  |  |  |  |  |  |  | 0 | 0 | $ 7,752 |  |  |
Secured Debt [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term Debt, Gross |  |  |  |  |  |  |  |  |  | $ 562,695 | 655,934 |  |  |  |
Encino Trace - Building I [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Construction in Progress, Gross |  |  |  |  |  |  |  |  |  |  | 38,800 |  |  |  |
Broadmoor Austin Associates [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acreage of Land | a |  |  | 66.0 |  |  |  |  |  |  |  |  |  |  |  |
Payments to Acquire Land |  |  | $ 143,800 |  |  |  |  |  |  |  |  |  |  |  |
Amount funded by corporate funds |  |  | $ 81,000 |  |  |  |  |  |  |  |  |  |  |  |
Percentage of ownership interests |  |  | 50.00% |  |  |  |  |  |  |  |  |  |  |  |
Number of Units in Real Estate Property | property |  |  | 7 |  |  |  |  |  |  |  |  |  |  |  |
Purchase Price of Assets |  |  | $ 211,400 |  |  |  |  |  |  |  |  |  |  |  |
Carrying amount of Investment |  |  | $ 66,300 |  |  |  |  |  |  |  |  |  |  |  |
Rentable Square Feet | ft² |  |  | 1,112,236 |  |  |  |  |  |  |  |  |  |  |  |
Lease percentage of acquired property |  |  | 100.00% |  |  |  |  |  |  |  |  |  |  |  |
Funds Held in Escrow |  |  | $ 62,800 |  |  |  |  |  |  |  |  |  |  |  |
Mortgage Debt Paid at Closing |  |  | 51,200 |  |  |  |  |  |  |  |  |  |  |  |
Acquisition Costs |  |  | $ 200 |  |  |  |  |  |  |  | $ 200 |  |  |  |
Ownership percentage |  |  | 50.00% |  |  |  |  |  |  |  |  |  |  |  |
Net gain from remeasurement of investments in real estate ventures |  |  | $ 800 |  |  |  |  |  |  |  |  |  |  |  |
Building, land and improvements |  |  | 163,271 |  |  |  |  |  |  |  |  |  |  |  |
Business Combination, Cash Payment at Settlement |  |  | $ 143,821 |  |  |  |  |  |  |  |  |  |  |  |
A618 Market Street |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to Acquire Land |  |  |  |  |  | $ 19,400 |  |  |  |  |  |  |  |  |
Area of Real Estate Property | ft² |  |  |  |  |  | 14,404 |  |  |  |  |  |  |  |  |
Number of Parking Spaces | ft² |  |  |  |  |  | 330 |  |  |  |  |  |  |  |  |
Fair Value of Contingent Consideration |  |  |  |  |  | $ 1,600 |  |  |  |  |  |  |  |  |
Consideration, Cash |  |  |  |  |  | 17,800 |  |  |  |  |  |  |  |  |
Building, land and improvements |  |  |  |  |  | 19,200 |  |  |  |  |  |  |  |  |
Intangible Assets |  |  |  |  |  | $ 200 |  |  |  |  |  |  |  |  |
Encino Trace - Land [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acreage of Land | a |  |  |  |  |  |  | 54.1 |  |  |  |  |  |  |  |
Payments to Acquire Land |  |  |  |  |  |  | $ 14,000 |  |  |  |  |  |  |  |
Rentable Square Feet | ft² |  |  |  |  |  |  | 320,000 |  |  | 320,000 |  |  |  |  |
Lease percentage of acquired property |  |  |  |  |  |  | 75.00% |  |  |  |  |  |  |  |
Funds Held in Escrow |  |  |  |  |  |  | $ 1,000 |  |  |  |  |  |  |  |
Construction in Progress, Gross |  |  |  |  |  |  | 8,400 |  |  |  |  |  |  |  |
Development in Progress |  |  |  |  |  |  | $ 4,600 |  |  |  |  |  |  | $ 4,600 |
Number of potential development properties | property |  |  |  |  |  |  | 2 |  |  |  |  |  |  |  |
Number of floors of a building |  |  |  |  |  |  | 4 |  |  | 4 |  |  |  |  |
Commerce Square [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acreage of Land | ft² |  |  |  |  |  |  |  | 1,896,142 |  |  |  |  |  |  |
Percentage of ownership interests |  |  |  |  |  |  |  |  |  | 99.00% |  |  |  |  |
Lease percentage of acquired property |  |  |  |  |  |  |  |  |  |  |  | 86.70% |  |  |
Acquisition Costs |  |  |  |  |  |  |  | $ 900 |  |  |  |  |  |  |
Percent of Acquired Entity |  |  |  |  |  |  |  | 99.00% |  |  |  |  |  |  |
Consideration, Cash |  |  |  |  |  |  |  | $ 331,800 |  |  |  |  |  |  |
Building, land and improvements |  |  |  |  |  |  |  | 255,705 |  |  |  |  |  |  |
Business Combination, Assumption of Existing Mortgage Debt |  |  |  |  |  |  |  | 237,100 |  |  |  |  |  |  |
Business Combination, Cash Payment at Settlement |  |  |  |  |  |  |  | 73,075 |  |  |  |  |  |  |
Net loss from remeasurement of investment in real estate ventures |  |  |  |  |  |  |  | 1,000 |  |  |  |  |  |  |
One Commerce Square [Member] | Secured Debt [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term Debt, Gross |  |  |  |  |  |  |  | $ 125,100 |  |  |  |  | $ 130,000 |  |
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate |  |  |  |  |  |  |  | 5.67% |  |  |  |  |  |  |
Long-term Debt, Fair Value |  |  |  |  |  |  |  | $ 130,200 |  |  |  |  |  |  |
Debt Instrument Maturity Date |  |  |  |  |  |  |  | Jan. 06, 2016 |  | Apr. 05, 2023 |  |  |  |  |
Two Commerce Square [Member] | Secured Debt [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term Debt, Gross |  |  |  |  |  |  |  | $ 112,000 |  |  |  |  |  |  |
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate |  |  |  |  |  |  |  | 3.96% |  |  |  |  |  |  |
Long-term Debt, Fair Value |  |  |  |  |  |  |  | $ 107,900 |  |  |  |  |  |  |
Debt Instrument Maturity Date |  |  |  |  |  |  |  |  |  | Apr. 05, 2023 |  |  |  |  |
Four Points Centre [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acreage of Land | a |  |  |  |  |  |  |  | 22.3 |  |  |  |  |  |  |
Rentable Square Feet | ft² |  |  |  |  |  |  |  | 192,396 |  |  |  |  |  |  |
Lease percentage of acquired property |  |  |  |  |  |  |  | 99.20% |  |  |  |  |  |  |
Acquisition Costs |  |  |  |  |  |  |  | $ 100 |  |  |  |  |  |  |
Consideration, Cash |  |  |  |  |  |  |  | $ 47,300 |  |  |  |  |  |  |
Number of floors of a building | Storey |  |  |  |  |  |  |  | 3 |  |  |  |  |  |  |
Business Acquisition Purchase Price Allocation Building |  |  |  |  |  |  |  | $ 36,000 |  |  |  |  |  |  |
Business Acquisition Purchase Price Allocation Land Portion |  |  |  |  |  |  |  | 5,800 |  |  |  |  |  |  |
Business Acquisition Purchase Price Allocation Intangible Assets |  |  |  |  |  |  |  | 6,500 |  |  |  |  |  |  |
Business Acquisition Purchase Price Allocated To Market Lease Liabilities |  |  |  |  |  |  |  | $ 1,000 |  |  |  |  |  |  |
Cira Ground Land Parcel [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acreage of Land | a |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0.8
Acquisition Costs |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,400
Ground Lease Buy-out, Total Cost |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 24,600
Accrued Rent |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 12,100
Three Logan Square [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acreage of Land | a |  |  |  |  |  |  |  |  | 1.8 |  |  |  |  |  |
Acquisition Costs |  |  |  |  |  |  |  |  | $ 100 |  |  |  |  |  |
Ground Lease Buy-out, Total Cost |  |  |  |  |  |  |  |  | 20,800 |  |  |  |  |  |
Change in operating real estate due to non-cash adjustment to land |  |  |  |  |  |  |  |  | $ 4,300 |  |  |  |  |  |
2100 Market Street [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acreage of Land | a |  | 0.8 |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to Acquire Land |  | $ 18,800 |  |  |  |  |  |  |  |  |  |  |  |  |
Amount funded by corporate funds |  | 16,800 |  |  |  |  |  |  |  |  |  |  |  |  |
Amount Deferred |  | $ 2,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Settlement Period |  | 24 months |  |  |  |  |  |  |  |  |  |  |  |  |
25 M Street [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acreage of Land | a |  |  |  |  | 0.8 |  |  |  |  |  |  |  |  |  |
Payments to Acquire Land |  |  |  |  | $ 20,300 |  |  |  |  |  |  |  |  |  |
Acquisition Costs |  |  |  |  | $ 300 |  |  |  |  |  |  |  |  |  |
Area of Real Estate Property | ft² |  |  |  | 271,000 |  |  |  |  |  |  |  |  |  |  |
25 M Street [Member] | Brandywine 25 M Street [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percent of Acquired Entity |  |  |  | 95.00% |  |  |  |  |  |  |  |  |  |  |
25 M Street [Member] | Jaco25 M Investors L L C |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to Acquire Land |  |  |  | $ 1,000 |  |  |  |  |  |  |  |  |  |  |
Asset Acquisition Ownership Percent |  |  |  | 5.00% |  |  |  |  |  |  |  |  |  |  |
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- Definition The amount of property, plant, and equipment recognized as of the acquisition date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=56956791&loc=d3e4845-128472 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 55 -Paragraph 37 -URI http://asc.fasb.org/extlink&oid=35744172&loc=d3e2207-128464 + Details
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- Definition Fair value at acquisition-date of the equity interest in the acquiree held by the acquirer, immediately before the acquisition date for businesses combined in stages. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (g)(1) -URI http://asc.fasb.org/extlink&oid=56956515&loc=d3e1392-128463 + Details
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- Definition Percentage of voting equity interests acquired in a business combination achieved in stages, including equity interests in the acquiree held by the acquirer immediately before the acquisition date and acquired at the acquisition date. + References No definition available. + Details
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- Definition In a business combination achieved in stages, this element represents the amount of gain recognized by the entity as a result of remeasuring to fair value the equity interest in the acquiree it held before the business combination. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (g)(2) -URI http://asc.fasb.org/extlink&oid=56956515&loc=d3e1392-128463 + Details
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- Definition In a business combination achieved in stages, this element represents the amount of loss recognized by the entity as a result of remeasuring to fair value the equity interest in the acquiree it held before the business combination. + References No definition available. + Details
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- Definition Amount of structure or a modification to a structure under construction. Includes recently completed structures or modifications to structures that have not been placed into service. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229 + Details
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- Definition Amount of long-term debt before deduction of unamortized discount or premium. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt, with initial maturities beyond one year or the normal operating cycle, if longer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 20 -Article 5 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 16 -Article 9 + Details
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- Definition Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(2)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition The current amount of expenditures for a real estate project that has not yet been completed. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.10) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Phase -URI http://asc.fasb.org/extlink&oid=49177889 + Details
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- Definition The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6382943&loc=d3e33918-111571 + Details
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- Definition The designation of funds furnished by a borrower to a lender to assure future payments of the borrower's real estate taxes and insurance obligations with respect to a mortgaged property. Escrow deposits may be made for a variety of other purposes such as earnest money and contingent payments. This element excludes replacement reserves which are an escrow separately provided for within the US GAAP taxonomy. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.10) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
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- Definition Sum of the carrying amounts as of the balance sheet date of all investments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.1(h)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
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- Definition The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. + References No definition available. + Details
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- Definition The interest rate applicable to the portion of the carrying amount of long-term borrowings outstanding as of the balance sheet date, including current maturities, which accrues interest at a set, unchanging rate. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Net rentable area for properties owned. + References No definition available. + Details
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- Definition The cash outflow from the acquisition of real estate intended to generate income for the owner; excludes land acquired for use by the owner. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 13 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=56944662&loc=d3e3213-108585 + Details
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