Hedging Activities - Additional Information (Detail)	6 Months Ended
	Jun. 30, 2014
Derivatives And Hedging Activities [Line Items]
Fuel hedges expiration date	2016-03
2014 Projected
Derivatives And Hedging Activities [Line Items]
Percentage of projected fuel requirements	21.00%
Volume of fuel hedging	416,000,000
2015 Projected
Derivatives And Hedging Activities [Line Items]
Percentage of projected fuel requirements	19.00%
Volume of fuel hedging	742,000,000
2016 Projected
Derivatives And Hedging Activities [Line Items]
Percentage of projected fuel requirements	1.00%
Volume of fuel hedging	35,000,000