Schedule of Gain on Derivative Instruments (Detail) - Designated as Hedging Instrument - Cash Flow Hedging - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2017	[1]	Sep. 30, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Loss Recognized in AOCI on Derivatives (Effective Portion)	$ (6)			$ (5)
Loss Reclassified from AOCI into Fuel Expense	$ (24)	$ (2)		$ (197)

[1] The 2017 loss reclassified from AOCI into fuel expense represents hedge losses on December 2016 settled trades, but for which the associated fuel purchased in December was not consumed until January 2017.