Operating Leases - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2012
Operating Leased Assets [Line Items]
Term of non-cancellable pick-up truck leases (in years)	1 year
Renewal options for non-cancellable pick-up truck leases, maximum (in months)	72 months
Average pick-up truck lease term, minimum (in months)	28 months
Average pick-up truck lease term, maximum (in months)	36 months
Aggregate residual value guarantee	$ 48,630
Loss on disposal reserve	$ 256