Hedging Activities - Additional Information (Detail) - Entity [Domain] $ in Millions	6 Months Ended
Jun. 30, 2015 USD ($) gal	Dec. 31, 2014 USD ($)
Derivatives And Hedging Activities [Line Items]
Fuel hedges expiration date	2016-03
Collateral posted with fuel derivative counterparties	$ 181	$ 577
Fair value of derivative asset	$ 40	6
Cash Flow Hedging
Derivatives And Hedging Activities [Line Items]
Percentage of projection of European euro for hedging financial instruments for remainder of 2015	38.00%
Percentage of projection of European euro for hedging financial instruments for 2016	22.00%
Percentage of projection of British pound for hedging financial instruments for remainder of 2015	11.00%
Foreign Currency Derivatives Asset, Net
Derivatives And Hedging Activities [Line Items]
Fair value of derivative asset	$ 6
2015 Projected
Derivatives And Hedging Activities [Line Items]
Percentage of projection for hedging financial instruments	22.00%
Volume of fuel hedging \| gal	442,000,000
2016 Projected
Derivatives And Hedging Activities [Line Items]
Percentage of projection for hedging financial instruments	5.00%
Volume of fuel hedging \| gal	180,000,000
Fuel Derivative Instruments
Derivatives And Hedging Activities [Line Items]
Collateral posted with fuel derivative counterparties	$ 181	$ 577

Aggregate notional amount of derivative expressed in nonmonetary units. For example, the number of barrels specified in a fuel oil forward purchase contract.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 12

-Section 13

-Sentence Column B

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- Definition

Cash Collateral Posted For Derivatives Liability

+ References

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- Definition

Derivative Instrument Expiry Date

+ References

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- Definition

Derivatives And Hedging Activities [Line Items]

+ References

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- Definition

Hedging contract as percentage of projected British pound net cash inflow

+ References

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- Definition

The percentage of projected European euro denominated net cash inflows that are hedged with foreign currency derivatives

+ References

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- Definition

Hedging contract as percentage of projected European euro net cash inflow in year two.

+ References

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- Definition

Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 20

-Section 50

-Paragraph 3

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4B

-Subparagraph (a),(c)

-URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5624163-113959

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 20

-Section 55

-Paragraph 7

-URI http://asc.fasb.org/extlink&oid=51825145&loc=SL20226000-175313

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 860

-SubTopic 20

-Section 50

-Paragraph 4D

-Subparagraph (c)(2)

-URI http://asc.fasb.org/extlink&oid=51814546&loc=SL51823488-111719

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- Definition

The percentage of expected requirements covered by the aggregate notional amount of derivative contracts expressed in nonmonetary units. For example, the number of barrels specified in a fuel oil forward purchase contract as a percentage of expected need.

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