Real Estate Facilities (Narrative) (Details)			6 Months Ended
	Feb. 27, 2015 USD ($) ft² item	Feb. 13, 2015 USD ($) ft² item	Jun. 30, 2015 USD ($) ft² item	Dec. 31, 2014 USD ($) ft² item
Real Estate Facilities [Line Items]
Land and building held for development			$ 26,288,000	$ 24,442,000
Northern Virginia Redeveloped Land [Member]
Real Estate Facilities [Line Items]
Area of redevelopment of real estate property | ft²			125,000
Economic interest in joint venture, percentage			95.00%
Land and building held for development			$ 20,200,000	$ 18,400,000
Capitalized cost			1,800,000
Capitalized interest expense			$ 531,000
Overlake [Member]
Real Estate Facilities [Line Items]
Disposal date of real estate			Feb. 27, 2015
Leasable space (in square feet) | ft²	82,000
Proceeds from the sale of real estate facility	$ 13,900,000
Number of buildings sold | item	5
Number of buildings held for disposition | item				5
Gain on sale of real estate facilities	$ 4,800,000
Milwaukie Business Park [Member]
Real Estate Facilities [Line Items]
Disposal date of real estate			Feb. 13, 2015
Leasable space (in square feet) | ft²		102,000
Proceeds from the sale of real estate facility		$ 10,600,000
Number of buildings sold | item		6
Gain on sale of real estate facilities		$ 7,600,000
Sacramento Business Parks [Member]
Real Estate Facilities [Line Items]
Leasable space (in square feet) | ft²			367,000	367,000
Number of business parks held for disposition | item			2	2
Sacramento Business Parks [Member] | Subsequent Event [Member]
Real Estate Facilities [Line Items]
Number of business parks sold | item			2
Proceeds from the sale of real estate facility			$ 29,300,000
Tempe Business Park [Member]
Real Estate Facilities [Line Items]
Leasable space (in square feet) | ft²			23,000	23,000
Number of properties held for disposition | item			1	1
Maximum [Member] | Northern Virginia Redeveloped Land [Member]
Real Estate Facilities [Line Items]
Leasable space (in square feet) | ft²			450,000