Assets Held for Sale by Segment (Details) - Operating Segments [Member] - 2016 held for sale [Member] - Disposal Group, Held-for-sale, Not Discontinued Operations [Member] ft² in Thousands	Sep. 30, 2016 ft² a bldg
Land and Land Improvements [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Area of Land | a	31.3
Building and Building Improvements [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Real Estate Properties | bldg	108
Net Rentable Area | ft²	7,579
Minnesota [Member] | Land and Land Improvements [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Area of Land | a	0.0
Minnesota [Member] | Building and Building Improvements [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Real Estate Properties | bldg	19
Net Rentable Area | ft²	1,489
Florida [Member] | Land and Land Improvements [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Area of Land | a	13.2
Florida [Member] | Building and Building Improvements [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Real Estate Properties | bldg	45
Net Rentable Area | ft²	2,935
Southeastern PA [Member] | Land and Land Improvements [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Area of Land | a	0.0
Southeastern PA [Member] | Building and Building Improvements [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Real Estate Properties | bldg	30
Net Rentable Area | ft²	2,076
Other Segments [Member] | Land and Land Improvements [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Area of Land | a	18.1
Other Segments [Member] | Building and Building Improvements [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Real Estate Properties | bldg	14
Net Rentable Area | ft²	1,079