Segment Information Building Acquisitions and Dispositions (Details) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2016 USD ($) ft² a bldg	Jun. 30, 2016 USD ($) ft² a bldg	Jun. 30, 2015 USD ($)
Property, Plant and Equipment [Line Items]
Proceeds from Sale of Real Estate Held-for-investment | $		$ 139,814	$ 156,891
Operating Segments [Member] | 2016 sales [Member] | Disposal Group, Not Discontinued Operations [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	9	9
Net Rentable Area | ft²	1,047,000	1,047,000
Proceeds from Sale of Real Estate Held-for-investment | $		$ 143,128
Operating Segments [Member] | 2016 sales [Member] | Disposal Group, Not Discontinued Operations [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 Real Estate Held-for-investment | $		$ 9,200
Operating Segments [Member] | 2016 sales [Member] | Disposal Group, Not Discontinued Operations [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 Real Estate Held-for-investment | $		$ 111,828
Operating Segments [Member] | 2016 sales [Member] | Disposal Group, Not Discontinued Operations [Member] | Florida [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Area of Land | a	3.5	3.5
Operating Segments [Member] | 2016 sales [Member] | Disposal Group, Not Discontinued Operations [Member] | Other Segments [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	2	2
Net Rentable Area | ft²	381,000	381,000
Proceeds from Sale of Real Estate Held-for-investment | $		$ 22,100
Operating Segments [Member] | 2016 sales current [Member] | Disposal Group, Not Discontinued Operations [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	1	1
Net Rentable Area | ft²	198,000	198,000
Proceeds from Sale of Real Estate Held-for-investment | $	$ 12,000
Operating Segments [Member] | 2016 sales current [Member] | Disposal Group, Not Discontinued Operations [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 Real Estate Held-for-investment | $	$ 0
Operating Segments [Member] | 2016 sales current [Member] | Disposal Group, Not Discontinued Operations [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 Real Estate Held-for-investment | $	$ 0
Operating Segments [Member] | 2016 sales current [Member] | Disposal Group, Not Discontinued Operations [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	198,000	198,000
Proceeds from Sale of Real Estate Held-for-investment | $	$ 12,000
Operating Segments [Member] | 2016 sales current [Member] | Disposal Group, Held-for-sale, Not Discontinued Operations [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
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
Net Rentable Area | ft²	73,000	73,000
Payments to Acquire Commercial Real Estate | $		$ 8,000