Hedging Activities - Additional Information (Detail)	6 Months Ended
	Jun. 30, 2013
Derivatives And Hedging Activities [Line Items]
Fuel hedges expiration date	2014-12
2013 Projected
Derivatives And Hedging Activities [Line Items]
Percentage of projected fuel requirements	47.00%
Volume of fuel hedging	942,000,000
2014 Projected
Derivatives And Hedging Activities [Line Items]
Percentage of projected fuel requirements	18.00%
Volume of fuel hedging	706,000,000