Real Estate Properties Disposition Acitivities (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016 USD ($) ft² building $ / shares	Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($) ft² building $ / shares	Jun. 30, 2015 USD ($)	Jul. 22, 2016 building	Dec. 31, 2015 USD ($)
Real estate properties
Number of Buildings | building	92		92
Net book value	$ 1,501,190		$ 1,501,190			$ 1,440,253
Balance Sheets:
Assets of property held for sale	3,095		3,095			3,098
Assets of discontinued operations	12,482		12,482			12,468
Liabilities of property held for sale	12		12			43
Liabilities of discontinued operations	83		83			54
Statement of Operations:
Rental income			56	$ 58
Real estate taxes			(46)	(140)
Utility expenses			(79)	(78)
Other operating expenses			(149)	(162)
General and administrative			(57)	(57)
Income (loss) from discontinued operations	$ (126)	$ (173)	$ (275)	$ (379)
Sacramento, CA
Real estate properties
Number of Buildings | $ / shares	1		1
Disposal Group, By Sale, Not Discontinued Operations | Savannah GA | London Interbank Offered Rate (LIBOR) [Member]
Real estate properties
Interest rate premium (as a percent)			4.00%
Disposal Group, By Sale, Not Discontinued Operations | Savannah GA | Minimum | London Interbank Offered Rate (LIBOR) [Member]
Real estate properties
Interest rate premium (as a percent)			5.00%
Continuing operations.
Balance Sheets:
Real estate properties, net	$ 3,071		$ 3,071			3,071
Rents receivable						1
Other assets	24		24			26
Assets of property held for sale	3,095		3,095			3,098
Other liabilities	12		12			43
Liabilities of property held for sale	12		12			43
Discontinued operations
Balance Sheets:
Real estate properties, net	12,260		12,260
Other assets	222		222
Assets of discontinued operations	12,482		12,482
Other liabilities	83		83
Liabilities of discontinued operations	83		$ 83
Statement of Operations:
Rental income	28	27
Real estate taxes	(23)	(70)
Utility expenses	(29)	(11)
Other operating expenses	(73)	(91)
General and administrative	(29)	(28)
Income (loss) from discontinued operations	$ (126)	$ (173)
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
Number of Buildings | building					1
Rentable square feet of properties | ft²	35,228		35,228
Net book value	$ 3,071		$ 3,071
Aggregate sale price of properties sold, excluding closing costs	$ 4,000		4,000
Mortgage financing to the buyer			$ 3,600
One building | Discontinued Operations, Held-for-sale | Office | Falls Church, VA
Real estate properties
Number of Buildings | building	1		1
Rentable square feet of properties | ft²	164,746		164,746
Net book value	$ 12,282		$ 12,282
Aggregate sale price of properties sold, excluding closing costs	$ 13,000		$ 13,000