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Derivatives - Summary of Fair Value of Open Commodity Derivatives (Detail) (Commodity derivatives [Member], USD $)
In Thousands, unless otherwise specified |
Dec. 31, 2012
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Dec. 31, 2011
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Derivative not designated as hedging instruments, fair value of liability derivative [Member]
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| Derivatives, Fair Value [Line Items] | ||
| Unrealized loss on commodity derivatives | $ 1,441 | |
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Derivative not designated as hedging instruments, fair value of assets derivative [Member]
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| Derivatives, Fair Value [Line Items] | ||
| Unrealized gain on commodity derivatives | $ 2,433 |