Property and Equipment - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Property, Plant and Equipment [Line Items]
Amortization of assets held under capital leases	$ 28.5	$ 22.0	$ 21.2
Maximum
Property, Plant and Equipment [Line Items]
Lease Obligation Period	5 years