Accounting for the Impairment or Disposal of Long-Lived Assets Schedule of Significant Acquisitions and Disposals (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013 sqft properties
Portfolio Sale, Total [Member]
Significant Acquisitions and Disposals [Line Items]
Sale Proceeds	$ 697.3
Number of Real Estate Properties	97
Square Footage of Real Estate Property	6,600,000
Portfolio Sale, first settlement [Member]
Significant Acquisitions and Disposals [Line Items]
Sale Proceeds	$ 367.7
Number of Real Estate Properties	49
Square Footage of Real Estate Property	4,000,000
Area of Land	140