Real Estate Investments Acquisitions (Details) $ in Thousands									3 Months Ended		9 Months Ended
		Aug. 11, 2015 USD ($) a	Jul. 07, 2015 USD ($) a	Jun. 22, 2015 USD ($) a ft² property	May. 12, 2015 USD ($) ft²	Apr. 09, 2015 USD ($) a	Apr. 06, 2015 USD ($) a	Apr. 02, 2015 USD ($) ft² parking_space	Sep. 30, 2015 USD ($) ft²	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) ft²	Sep. 30, 2014 USD ($)
Business Acquisition [Line Items]
Net gain from remeasurement of investment in real estate ventures									$ 0	$ 0	$ 758	$ 458
Net Rentable Space | parking_space								330
Broadmoor Austin Associates [Member]
Business Acquisition [Line Items]
Area Owned by Company of Undeveloped Parcels Of Land | a				66.0
Payments to Acquire Land				$ 143,800
Amount funded by corporate funds				$ 81,000
Percentage of ownership interests				50.00%
Purchase Price of Assets				$ 211,400
Rentable Square Feet | ft²				1,112,236
Lease Percentage of Acquired Property									100.00%		100.00%
Funds Held in Escrow				$ 62,800
Mortgage Debt Paid at Closing				$ 51,200
Ownership percentage				50.00%
Net gain from remeasurement of investment in real estate ventures				$ 800
Building, land and improvements				163,271
Broadmoor Austin Associates [Member] | General and Administrative Expense [Member]
Business Acquisition [Line Items]
Acquisition Costs				$ 200
618 Market Street [Member]
Business Acquisition [Line Items]
Payments to Acquire Land								$ 19,400
Total Potential Development Capacity | ft²								14,404
Fair Value of Contingent Consideration								$ 1,600
Consideration, Cash								17,800
Building, land and improvements								19,200
Intangible Assets								$ 200
Office Building [Member]
Business Acquisition [Line Items]
Rentable Square Feet | ft²	[1]								1,415,704		1,415,704
Office Building [Member] | Broadmoor Austin Associates [Member]
Business Acquisition [Line Items]
Number of Units in Real Estate Property | property				7
9 Presidential Boulevard [Member]
Business Acquisition [Line Items]
Area Owned by Company of Undeveloped Parcels Of Land | a		2.7
Payments to Acquire Land		$ 4,100
2100 Market Street [Member]
Business Acquisition [Line Items]
Area Owned by Company of Undeveloped Parcels 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
405 Colorado Drive [Member]
Business Acquisition [Line Items]
Area Owned by Company of Undeveloped Parcels Of Land | a						0.4
Payments to Acquire Land						$ 2,600
25 M Street [Member]
Business Acquisition [Line Items]
Area Owned by Company of Undeveloped Parcels Of Land | a							0.8
Payments to Acquire Land							$ 20,300
Acquisition Costs							$ 300
Total Potential Development Capacity | 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] | Akridge [Member]
Business Acquisition [Line Items]
Payments to Acquire Land					$ 1,000
Asset Acquisition Ownership Percent					5.00%

[1] Total gain/(loss) on sale does not include a deferred gain of $0.5 million related to a prior sale and $0.1 million in losses associated with prior sales.