Annual report pursuant to Section 13 and 15(d)

Selected Quarterly Financial Data - Additional Information (Detail)

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Selected Quarterly Financial Data - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
3 Months Ended 12 Months Ended
Dec. 31, 2013
MMcf
Dec. 31, 2013
MMcf
Quarterly Financial Data [Line Items]    
One-time increase in production 2,600 1,900
Natural Gas
   
Quarterly Financial Data [Line Items]    
Adjustment to depreciation, depletion, amortization and accretion $ 7.1 $ 5.0
Reduction in net income $ 4.6 $ 3.2