Real Estate Investments - Acquisitions (Textual) (Details) $ in Thousands										12 Months Ended
		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%

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