Real Estate Investments Dispositions (Details) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2015 USD ($) a ft² property parcel	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) a ft² property parcel	Sep. 30, 2014 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain/(Loss) On Sale		$ 6,083	$ 4,698	$ 16,673	$ 4,698
Provision for impairment on assets held for sale		0	$ 1,765	2,508	1,765
Gain (loss) on sale of properties				(19,692)	$ (5,882)
Sold Property		206,400		206,400
Disposition Target				400,000
Land [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sale Price		$ 15,800		15,800
Gain/(Loss) On Sale				$ 3,019
Number of Parcels | parcel		3		3
Area Owned by Company of Undeveloped Parcels Of Land | a		13.1		13.1
Land [Member] | 7000 Midlantic [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sale Price		$ 2,200		$ 2,200
Gain/(Loss) On Sale				$ (169)
Number of Parcels | parcel		1		1
Area Owned by Company of Undeveloped Parcels Of Land | a		3.5		3.5
Land [Member] | Four Points Centre [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sale Price		$ 2,500		$ 2,500
Gain/(Loss) On Sale				$ 71
Number of Parcels | parcel		1		1
Area Owned by Company of Undeveloped Parcels Of Land | a		8.6		8.6
Land [Member] | Two Kaiser Plaza [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sale Price		$ 11,100		$ 11,100
Gain/(Loss) On Sale				$ 3,117
Number of Parcels | parcel		1		1
Area Owned by Company of Undeveloped Parcels Of Land | a		1.0		1.0
Office Building [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Properties | property	[1]	15		15
Rentable Square Feet | ft²	[1]	1,415,704		1,415,704
Sale Price	[1]	$ 206,425		$ 206,425
Gain/(Loss) On Sale	[1],[2]			16,320
Deferred gain		$ 500		500
Gain (loss) on sale of properties				$ 100
Office Building [Member] | 1000 Howard Boulevard [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Properties | property		1		1
Rentable Square Feet | ft²		105,312		105,312
Sale Price		$ 16,500		$ 16,500
Gain/(Loss) On Sale	[2]			$ 4,823
Office Building [Member] | Bay Colony [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Properties | property		4		4
Rentable Square Feet | ft²		247,294		247,294
Sale Price		$ 37,500		$ 37,500
Gain/(Loss) On Sale	[2]			$ 288
Office Building [Member] | 741 First Avenue [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Properties | property		1		1
Rentable Square Feet | ft²		77,184		77,184
Sale Price		$ 4,900		$ 4,900
Gain/(Loss) On Sale	[2]			$ 372
Office Building [Member] | 100 Gateway Centre Parkway [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Properties | property	[3]	1		1
Rentable Square Feet | ft²	[3]	74,991		74,991
Sale Price	[3]	$ 4,100		$ 4,100
Gain/(Loss) On Sale	[2],[3]			0
Provision for impairment on assets held for sale				$ 800
Office Building [Member] | Christina / Delaware Corporate Centers [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Properties | property		5		5
Rentable Square Feet | ft²		485,182		485,182
Sale Price		$ 50,125		$ 50,125
Gain/(Loss) On Sale	[2]			$ 1,797
Office Building [Member] | Lake Merritt Tower [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Properties | property	[4]	1		1
Rentable Square Feet | ft²	[4]	204,336		204,336
Sale Price	[4]	$ 65,000		$ 65,000
Gain/(Loss) On Sale	[2],[4]			0
Provision for impairment on assets held for sale				$ 1,700
Office Building [Member] | Atrium I / Libertyview [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Properties | property		2		2
Rentable Square Feet | ft²		221,405		221,405
Sale Price		$ 28,300		$ 28,300
Gain/(Loss) On Sale	[2]			$ 9,040
Assets Held-for-sale [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Properties | property		6		6
Rentable Square Feet | ft²		560,147		560,147
Contractual Sales Price		$ 56,500		$ 56,500

[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.

[2] Gain/(Loss) on Sale is net of closing and other transaction related costs.

[3] The Company recorded an impairment loss of $0.8 million for 100 Gateway Centre Parkway during the second quarter of 2015. As such, there was no gain/(loss) at disposition for this property.

[4] The Company recorded an impairment loss of $1.7 million for Lake Merritt Tower at March 31, 2015. As such, there was no gain/(loss) at disposition for this property. Sales proceeds were deposited in escrow under Section 1031 of the Internal Revenue Code and applied to purchase the Broadmoor Austin portfolio. Refer to Broadmoor Austin Associates acquisition summary, above, for further details.