Hedging Activities - Additional Information (Detail)	Dec. 31, 2012 gal
2013 Projected
Derivatives And Hedging Activities [Line Items]
Percentage of projected fuel requirements 2013	31.00%
Volume of fuel hedging for 2013	1,200,000,000
2014 Projected
Derivatives And Hedging Activities [Line Items]
Percentage of projected fuel requirements 2013	2.00%
Volume of fuel hedging for 2013	63,000,000

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

Name:	invest_DerivativeNonmonetaryNotionalAmount
Namespace Prefix:	invest_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant