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Derivative Instruments and Risk Management (Details) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2013
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Dec. 31, 2012
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Summary of commodity swaps    
Commodity swaps, notional quantity 100 275
Derivatives (Level 1 measurement) [Member] | Recurring [Member] | Carrying Amount [Member]
   
Summary of commodity swaps    
Commodity swaps $ 0 $ 0.1
Derivatives (Level 1 measurement) [Member] | Recurring [Member] | Fair Value [Member]
   
Summary of commodity swaps    
Commodity swaps $ 0 $ 0.1