Assets Held for Sale and Discontinued Operations - Additional Information (Detail)	9 Months Ended	12 Months Ended
	Sep. 30, 2012 sqft Property	Dec. 31, 2011 sqft Property
Assets Held For Sale [Line Items]
Properties classified as held for sale	23
Number of properties sold to third parties	113	94
Number of properties sold to third parties	14,400,000	10,700,000