Real Estate Properties Disposition Acitivities (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2016 USD ($) ft²	Mar. 31, 2015 USD ($)	Mar. 31, 2016 building	Mar. 31, 2016 item	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Real estate properties
Number of Buildings			92	92
Net book value					$ 1,501,794	$ 1,440,253
Balance Sheets:
Assets of property held for sale					3,098	3,098
Assets of discontinued operations					12,502	12,468
Liabilities of property held for sale					32	43
Liabilities of discontinued operations					75	54
Statement of Operations:
Income (loss) from discontinued operations	$ (149)	$ (206)
Continuing operations.
Balance Sheets:
Real estate properties, net					3,071	3,071
Rents receivable						1
Other assets					27	26
Assets of property held for sale					3,098	3,098
Other liabilities					32	43
Liabilities of property held for sale					32	43
Continuing operations. | Office | Savannah GA
Real estate properties
Number of Buildings | building			1
Discontinued operations
Balance Sheets:
Real estate properties, net					12,260
Rents receivable					17
Other assets					225
Assets of discontinued operations					12,502
Other liabilities					75
Liabilities of discontinued operations					75
Statement of Operations:
Rental income	28	31
Real estate taxes	(23)	(70)
Utility expenses	(50)	(67)
Other operating expenses	(76)	(71)
General and administrative	(28)	(29)
Income (loss) from discontinued operations	$ (149)	$ (206)
Discontinued operations | Discontinued Operations, Held-for-sale
Balance Sheets:
Real estate properties, net						12,260
Other assets						208
Assets of discontinued operations						12,468
Other liabilities						54
Liabilities of discontinued operations						$ 54
One building | Continuing operations. | Office | Savannah GA
Real estate properties
Rentable square feet of properties | ft²	35,228
Net book value					3,071
Option purchase price					4,500
One building | Discontinued Operations, Held-for-sale | Office | Falls Church, VA
Real estate properties
Number of Buildings | building			1
Rentable square feet of properties | ft²	164,746
Net book value					12,282
Aggregate sale price of properties sold, excluding closing costs					$ 14,750