Location and Amounts of Derivative Gains and Losses in Condensed Consolidated Statements of Income (Detail) (Derivatives Not Designated as Hedging Instruments, Commodity contracts, Net realized and unrealized gains (losses) on fuel price derivatives, USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Derivatives Not Designated as Hedging Instruments | Commodity contracts | Net realized and unrealized gains (losses) on fuel price derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain or (Loss) Recognized in Income on Derivative	$ 1,845	$ (7,755)