Offsetting Liabilities (Detail) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Fuel Derivative Instruments
Offsetting Asset and Liabilities [Line Items]
Potential net fair value positions elected to offset	$ (4)	$ (98)

Fair value as of the balance sheet date of price risk derivative assets, net of price risk derivative liabilities, which includes all such derivative instruments in hedging and nonhedging relationships that are recognized on the balance sheet.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13433-108611

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13495-108611

Name:	us-gaap_PriceRiskDerivativesAtFairValueNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant