Assets Held for Sale and Discontinued Operations - Additional Information (Detail)	3 Months Ended	12 Months Ended
	Mar. 31, 2013 sqft Property	Dec. 31, 2012 sqft Property
Assets Held For Sale [Line Items]
Operating property classified as held for sale	1
Number of properties sold to third parties	7	200
Number of properties sold to third parties (Area)	400,000	27,200,000