Hedging Activities - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Derivatives And Hedging Activities [Line Items]
Fuel hedges expiration date	2016-03
Collateral fuel derivative	$ 577
2015 Projected
Derivatives And Hedging Activities [Line Items]
Percentage of projected fuel requirements	22.00%
Volume of fuel hedging	859,000,000
2016 Projected
Derivatives And Hedging Activities [Line Items]
Percentage of projected fuel requirements	1.00%
Volume of fuel hedging	35,000,000
Fuel Derivative Instruments
Derivatives And Hedging Activities [Line Items]
Collateral fuel derivative	$ 577	$ 0

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

Derivative Instrument Expiry Date

+ References

No definition available.

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- Definition

The aggregate fair value of assets that are already posted, at the end of the reporting period, as collateral for derivative instruments with credit-risk-related contingent features.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4H

-Subparagraph (d)

-URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624258-113959

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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.

+ References

No definition available.

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