Fuel Swap Agreements (Detail)	Dec. 31, 2011	Dec. 31, 2010 MetricTon
Fuel Swap Agreement 2011
Derivative [Line Items]
Fuel Swap Agreements (metric tons)		766,000
Fuel Swap Agreements (% hedged)	0.00%	58.00%
Fuel Swap Agreement 2012
Derivative [Line Items]
Fuel Swap Agreements (metric tons)	738,000	738,000
Fuel Swap Agreements (% hedged)	55.00%	55.00%
Fuel Swap Agreement 2013
Derivative [Line Items]
Fuel Swap Agreements (metric tons)	644,000	300,000
Fuel Swap Agreements (% hedged)	47.00%	22.00%
Fuel Swap Agreement 2014
Derivative [Line Items]
Fuel Swap Agreements (metric tons)	418,000
Fuel Swap Agreements (% hedged)	30.00%
Fuel Swap Agreement 2015
Derivative [Line Items]
Fuel Swap Agreements (metric tons)	284,000
Fuel Swap Agreements (% hedged)	20.00%