Building Acquisitions and Dispositions (Details) $ in Thousands		3 Months Ended	9 Months Ended
		Sep. 30, 2016 USD ($) ft² a bldg	Sep. 30, 2016 USD ($) ft² a bldg	Sep. 30, 2015 USD ($)
Property, Plant and Equipment [Line Items]
Proceeds from Sale of Buildings and Land			$ 142,808	$ 243,943
Disposal Group, Not Discontinued Operations [Member] | 2016 sales current quarter [Member] | Operating Segments [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg		1	1
Net Rentable Area | ft²		80,000	80,000
Proceeds from Sale of Buildings and Land		$ 4,391
Disposal Group, Not Discontinued Operations [Member] | 2016 sales current quarter [Member] | Operating Segments [Member] | Minnesota [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg		0	0
Net Rentable Area | ft²		0	0
Proceeds from Sale of Buildings and Land		$ 0
Disposal Group, Not Discontinued Operations [Member] | 2016 sales current quarter [Member] | Operating Segments [Member] | Florida [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg		0	0
Net Rentable Area | ft²		0	0
Proceeds from Sale of Buildings and Land		$ 0
Disposal Group, Not Discontinued Operations [Member] | 2016 sales current quarter [Member] | Operating Segments [Member] | Other Segments [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg		1	1
Net Rentable Area | ft²		80,000	80,000
Proceeds from Sale of Buildings and Land		$ 4,391
Disposal Group, Not Discontinued Operations [Member] | 2016 sales [Member] | Operating Segments [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg		10	10
Net Rentable Area | ft²		1,127,000	1,127,000
Proceeds from Sale of Buildings and Land			$ 147,519
Disposal Group, Not Discontinued Operations [Member] | 2016 sales [Member] | Operating Segments [Member] | Minnesota [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg		1	1
Net Rentable Area | ft²		92,000	92,000
Proceeds from Sale of Buildings and Land			$ 9,200
Disposal Group, Not Discontinued Operations [Member] | 2016 sales [Member] | Operating Segments [Member] | Florida [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Area of Land | a		3.5	3.5
Disposal Group, Not Discontinued Operations [Member] | 2016 sales [Member] | Operating Segments [Member] | Florida [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg		6	6
Net Rentable Area | ft²		574,000	574,000
Proceeds from Sale of Buildings and Land	[1]		$ 111,828
Disposal Group, Not Discontinued Operations [Member] | 2016 sales [Member] | Operating Segments [Member] | Other Segments [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg		3	3
Net Rentable Area | ft²		461,000	461,000
Proceeds from Sale of Buildings and Land			$ 26,491
2016 acquisitions [Member] | Operating Segments [Member] | Chicago Milwaukee [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg		1	1
Payments to Acquire Commercial Real Estate			$ 8,000
Net Rentable Area | ft²		73,000	73,000

[1] Includes gross proceeds from the sale of 3.5 acres of land.