Accounting for the Impairment or Disposal of Long-Lived Assets Accounting for the Impairment or Disposal of Long-Lived Assets - Assets Held for Sale (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012 sqft properties
Long Lived Assets Held-for-sale [Line Items]
Assets Held-for-sale, Long Lived	200.8
Long Lived Assets Held-for-sale, Proceeds from Sale	199.1
Real Estate Properties Held-for-sale [Member] | Central Region [Member]
Long Lived Assets Held-for-sale [Line Items]
Number Properties Held For Sale	18
Property Square Feet Held For Sale	957,000
Real Estate Properties Held-for-sale [Member] | South Region [Member]
Long Lived Assets Held-for-sale [Line Items]
Number Properties Held For Sale	16
Property Square Feet Held For Sale	980,000
Real Estate Properties Held-for-sale [Member] | Northeast Other [Member]
Long Lived Assets Held-for-sale [Line Items]
Number Properties Held For Sale	16
Property Square Feet Held For Sale	633,000
Sold as of financial statement release date [Member]
Long Lived Assets Held-for-sale [Line Items]
Number Properties Held For Sale	49
Long Lived Assets Held-for-sale, Proceeds from Sale	195.0