Quantitative Information About Level Three Fair Value Measurements (Detail) (Fuel price derivatives - diesel, USD $) In Thousands, unless otherwise specified					3 Months Ended
	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2014 Option Model Minimum	Mar. 31, 2014 Option Model Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value	$ (623)	$ (2,142)	$ (929)	$ (107)
Future retail price of diesel (in dollars per gallon)					3.71	3.87