Real Estate Facilities (Narrative) (Details)						1 Months Ended	12 Months Ended
	Sep. 28, 2015 USD ($) ft²	Feb. 27, 2015 USD ($) ft² item	Feb. 13, 2015 USD ($) ft² item	Nov. 21, 2014 USD ($) ft² item	Oct. 01, 2014 USD ($) a ft² item	Jul. 31, 2015 USD ($) ft² item	Dec. 31, 2015 USD ($) ft²	Dec. 31, 2014 USD ($) ft²	Dec. 31, 2013 USD ($)	Jul. 24, 2015 ft² item	Dec. 30, 2014 USD ($) ft² item	Nov. 03, 2014 USD ($) ft²	Aug. 21, 2014 USD ($) ft² item	Jul. 28, 2014 USD ($) ft²	Jul. 24, 2014 USD ($) ft²	Dec. 20, 2013 USD ($) ft² property	Nov. 08, 2013 USD ($) ft² property	Oct. 15, 2013 USD ($) a ft² property	Jul. 26, 2013 USD ($) ft² property
Real Estate Facilities [Line Items]
Federal income tax basis of real estate facilities							$ 1,900,000,000
Percentage of properties encumbered by mortgage debt							21.70%
Leasable space (in square feet) | ft²							27,969
Acquisition transaction costs								$ 350,000	$ 854,000
Area of property held for disposition | ft²								574,000
Charcot Business Park II [Member]
Real Estate Facilities [Line Items]
Acquisition date of real estate							Dec. 30, 2014
Number of buildings acquired | item											8
Leasable space (in square feet) | ft²											119,000
Purchase price											$ 16,000,000
Charcot Business Park [Member]
Real Estate Facilities [Line Items]
Leasable space (in square feet) | ft²							164,000
McNeil I [Member]
Real Estate Facilities [Line Items]
Acquisition date of real estate							Nov. 03, 2014
Leasable space (in square feet) | ft²												246,000
Purchase price												$ 10,600,000
Bayshore Corporate Center [Member]
Real Estate Facilities [Line Items]
Acquisition date of real estate							Dec. 20, 2013
Number of buildings acquired | property																8
Leasable space (in square feet) | ft²																340,000
Purchase price																$ 60,500,000
MICC Center 23 [Member]
Real Estate Facilities [Line Items]
Acquisition date of real estate							Jul. 24, 2014
Leasable space (in square feet) | ft²															149,000
Purchase price															$ 12,700,000
MICC [Member]
Real Estate Facilities [Line Items]
Leasable space (in square feet) | ft²							3,300,000
Arapaho Business Park 9 [Member]
Real Estate Facilities [Line Items]
Acquisition date of real estate							Jul. 28, 2014
Leasable space (in square feet) | ft²														19,000
Purchase price														$ 1,100,000
Springlake Business Center II [Member]
Real Estate Facilities [Line Items]
Acquisition date of real estate							Aug. 21, 2014
Number of buildings acquired | item													6
Leasable space (in square feet) | ft²													145,000
Purchase price													$ 5,100,000
Arapaho Business Park [Member]
Real Estate Facilities [Line Items]
Acquisition date of real estate							Jul. 26, 2013
Number of buildings acquired | property																			18
Leasable space (in square feet) | ft²							389,000												389,000
Purchase price																			$ 14,800,000
Dallas Multi-Tenant Flex Parks [Member]
Real Estate Facilities [Line Items]
Acquisition date of real estate							Oct. 15, 2013
Number of parks acquired | property																		4
Number of acres acquired | a																		4
Leasable space (in square feet) | ft²																		559,000
Purchase price																		$ 27,900,000
Valwood Business Center [Member]
Real Estate Facilities [Line Items]
Acquisition date of real estate							Nov. 08, 2013
Number of buildings acquired | property																	9
Leasable space (in square feet) | ft²																	245,000
Purchase price																	$ 12,400,000
McKellips Business Park [Member]
Real Estate Facilities [Line Items]
Leasable space (in square feet) | ft²	23,000
Disposal date of real estate							Sep. 28, 2015
Proceeds from the sale of real estate facility	$ 1,300,000
Gain on sale of real estate facilities	$ 759,000
North Pointe Business Park [Member]
Real Estate Facilities [Line Items]
Leasable space (in square feet) | ft²										213,000
Disposal date of real estate							Jul. 24, 2015
Number of buildings sold | item										5
Northgate Business Park [Member]
Real Estate Facilities [Line Items]
Leasable space (in square feet) | ft²						154,000
Disposal date of real estate							Jul. 31, 2015
Number of buildings sold | item						12
Overlake [Member]
Real Estate Facilities [Line Items]
Leasable space (in square feet) | ft²		82,000
Disposal date of real estate							Feb. 27, 2015
Proceeds from the sale of real estate facility		$ 13,900,000
Number of buildings sold | item		5
Gain on sale of real estate facilities		$ 4,800,000
Milwaukie Business Park [Member]
Real Estate Facilities [Line Items]
Leasable space (in square feet) | ft²			102,000
Disposal date of real estate							Feb. 13, 2015
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]
Disposal date of real estate							Jul. 01, 2015
Number of business parks sold | item						2
Proceeds from the sale of real estate facility						$ 29,300,000
Gain on sale of real estate facilities						$ 15,000,000
Phoenix Business Parks [Member]
Real Estate Facilities [Line Items]
Leasable space (in square feet) | ft²				656,000
Disposal date of real estate							Nov. 21, 2014
Number of business parks sold | item				3
Proceeds from the sale of real estate facility				$ 52,200,000
Number of buildings sold | item				42
Gain on sale of real estate facilities				$ 29,600,000
Beaverton Business Parks [Member]
Real Estate Facilities [Line Items]
Leasable space (in square feet) | ft²					1,200,000
Disposal date of real estate							Oct. 01, 2014
Number of acres | a					11.5
Number of business parks sold | item					2
Proceeds from the sale of real estate facility					$ 159,900,000
Number of buildings sold | item					18
Gain on sale of real estate facilities					$ 62,800,000