Hedging Activities - Additional Information (Detail)	3 Months Ended
	Mar. 31, 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	22.00%
Volume of fuel hedging	668,000,000
2015 Projected
Derivatives And Hedging Activities [Line Items]
Percentage of projected fuel requirements	16.00%
Volume of fuel hedging	650,000,000
2016 Projected
Derivatives And Hedging Activities [Line Items]
Percentage of projected fuel requirements	1.00%
Volume of fuel hedging	6,000,000