Real Estate Investments - Acquisitions (Textual) (Details) $ in Thousands								3 Months Ended								12 Months Ended
	Jul. 01, 2016 USD ($) a	Jul. 07, 2015 USD ($) 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 ($) ft² a property Floor	Dec. 31, 2016 USD ($) a	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($) ft² Storey	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2016 USD ($) a	Dec. 31, 2015 USD ($) ft² Storey	Dec. 31, 2014 USD ($)	Jan. 30, 2017 a	Jun. 30, 2014 USD ($)
Business Acquisition [Line Items]
Amount funded by corporate funds																$ 20,406	$ 150,472	$ 18,443
Net gain from remeasurement of investments in real estate ventures																0	758	458
Revenue								$ 132,086	$ 129,694	$ 127,181	$ 136,502	$ 153,992	$ 152,585	$ 145,648	$ 150,406	525,463	602,631	596,982
Net losses								(14,058)	$ 6,022	$ (3,105)	$ 44,091	(63,941)	$ 18,346	$ 1,255	$ 6,710	40,191	(30,401)	6,975
Development in Progress								$ 297,462				$ 268,983				$ 297,462	268,983	201,360
Encino Trace - Building I [Member]
Business Acquisition [Line Items]
Construction in Progress, Gross																		38,800
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%
Subsequent Event [Member] | Garza Land Sale [Member]
Business Acquisition [Line Items]
Area of land under agreement to sell | a																			1.7
Garza Land Acquisition [Member]
Business Acquisition [Line Items]
Acreage of land | 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
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 Property units | 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
Revenue																$ 26,400	13,200
Net losses																$ 5,200	$ 7,200
Building, land and improvements			$ 163,271
25 M Street [Member] | 25 M Street [Member]
Business Acquisition [Line Items]
Payments to Acquire Land				$ 1,000
Asset Acquisition Ownership Percent				5.00%
618 Market Street [Member]
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					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					4