Hedging Activities - Additional Information (Detail)	9 Months Ended
	Sep. 30, 2013
Derivatives And Hedging Activities [Line Items]
Fuel hedges expiration date	2015-03
2013 Projected
Derivatives And Hedging Activities [Line Items]
Percentage of projected fuel requirements	45.00%
Volume of fuel hedging	428,000,000
2014 Projected
Derivatives And Hedging Activities [Line Items]
Percentage of projected fuel requirements	23.00%
Volume of fuel hedging	901,000,000
2015 Projected
Derivatives And Hedging Activities [Line Items]
Percentage of projected fuel requirements	1.00%
Volume of fuel hedging	25,000,000