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Derivatives - Summary of Changes in Fair Value of Commodity Derivatives (Detail) (Commodity derivatives [Member], USD $)
In Thousands, unless otherwise specified |
3 Months Ended | |
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Mar. 31, 2013
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Mar. 31, 2012
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| Derivative Instruments, Gain (Loss) [Line Items] | ||
| Derivative not designated as hedging instruments, total gain (loss) | $ (3,793) | $ (3,156) |
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Unrealized (loss) gain on commodity derivatives [Member]
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| Derivative Instruments, Gain (Loss) [Line Items] | ||
| Unrealized loss on commodity derivatives | (4,100) | (2,672) |
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Realized gain on commodity derivatives [Member]
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| Derivative Instruments, Gain (Loss) [Line Items] | ||
| Realized gain (loss) on commodity derivatives | $ 307 | $ (484) |