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Asset Retirement Obligation - Additional Information (Detail)
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12 Months Ended | ||
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Dec. 31, 2013
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Dec. 31, 2012
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Dec. 31, 2011
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| Asset Retirement Obligations [Line Items] | |||
| Credit-adjusted risk-free discounted rate | 10.00% | 10.00% | 10.00% |
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Minimum
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| Asset Retirement Obligations [Line Items] | |||
| Credit-adjusted risk-free discounted rate | 7.00% | ||
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Maximum
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| Asset Retirement Obligations [Line Items] | |||
| Credit-adjusted risk-free discounted rate | 12.00% | ||