Segment Information Building Acquisitions and Dispositions (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2016 USD ($) ft² a bldg	Mar. 31, 2015 USD ($)
Property, Plant and Equipment [Line Items]
Proceeds from Sale of Real Estate Held-for-investment | $	$ 127,727	$ 40,998
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	8
Net Rentable Area | ft²	849,000
Proceeds from Sale of Real Estate Held-for-investment | $	$ 131,128
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
Net Rentable Area | ft²	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
Net Rentable Area | ft²	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
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	1
Net Rentable Area | ft²	183,000
Proceeds from Sale of Real Estate Held-for-investment | $	$ 10,100
Operating Segments [Member] | 2016 sales [Member] | Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Dallas [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	1
Net Rentable Area | ft²	198,000
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
Net Rentable Area | ft²	73,000
Payments to Acquire Commercial Real Estate | $	$ 8,000