Operating Leases - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2012 Month Year
Operating Leased Assets [Line Items]
Term of non-cancellable pick-up truck leases (in years)	1
Renewal options for non-cancellable pick-up truck leases, maximum (in months)	72
Average pick-up truck lease term, minimum (in months)	28
Average pick-up truck lease term, maximum (in months)	36
Aggregate residual value guarantee	$ 53,006
Loss on disposal reserve	$ 397