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Derivative Instruments and Risk Management - Summary of Commodity Swaps (Detail) (Derivatives (Level 1 Measurement) [Member], Recurring [Member], USD $)
In Millions, unless otherwise specified
Jun. 30, 2013
Dec. 31, 2012
Carrying Amount [Member]
   
Derivative [Line Items]    
Commodity swaps $ 0 $ 0.1
Fair Value [Member]
   
Derivative [Line Items]    
Commodity swaps $ 0 $ 0.1