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Summary of Significant Accounting Policies - Additional Information 1 (Detail) (USD $)
 3 Months Ended 12 Months Ended
Dec. 31, 2014
Sep. 30, 2014
Jun. 30, 2014
Mar. 31, 2014
Dec. 31, 2013
Sep. 30, 2013
Jun. 30, 2013
Mar. 31, 2013
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Summary Of Significant Accounting Policies [Line Items]                      
Net book value of unproved properties $ 943,200,000us-gaap_UnprovedOilAndGasPropertySuccessfulEffortMethod       $ 807,000,000us-gaap_UnprovedOilAndGasPropertySuccessfulEffortMethod       $ 943,200,000us-gaap_UnprovedOilAndGasPropertySuccessfulEffortMethod $ 807,000,000us-gaap_UnprovedOilAndGasPropertySuccessfulEffortMethod  
Abandonment and impairment of unproved properties 14,308,000us-gaap_ExplorationAbandonmentAndImpairmentExpense 13,444,000us-gaap_ExplorationAbandonmentAndImpairmentExpense 9,332,000us-gaap_ExplorationAbandonmentAndImpairmentExpense 9,995,000us-gaap_ExplorationAbandonmentAndImpairmentExpense 5,852,000us-gaap_ExplorationAbandonmentAndImpairmentExpense 11,692,000us-gaap_ExplorationAbandonmentAndImpairmentExpense 19,156,000us-gaap_ExplorationAbandonmentAndImpairmentExpense 15,218,000us-gaap_ExplorationAbandonmentAndImpairmentExpense 47,079,000us-gaap_ExplorationAbandonmentAndImpairmentExpense 51,918,000us-gaap_ExplorationAbandonmentAndImpairmentExpense 125,278,000us-gaap_ExplorationAbandonmentAndImpairmentExpense
Useful life of assets                 10 years    
Depreciation expense                 12,900,000us-gaap_Depreciation 13,200,000us-gaap_Depreciation 13,200,000us-gaap_Depreciation
Unamortized debt issuance costs 42,200,000us-gaap_UnamortizedDebtIssuanceExpense       44,500,000us-gaap_UnamortizedDebtIssuanceExpense       42,200,000us-gaap_UnamortizedDebtIssuanceExpense 44,500,000us-gaap_UnamortizedDebtIssuanceExpense  
Marketable securities 68,500,000us-gaap_MarketableSecurities       67,800,000us-gaap_MarketableSecurities       68,500,000us-gaap_MarketableSecurities 67,800,000us-gaap_MarketableSecurities  
Other investments $ 10,300,000us-gaap_OtherInvestments       $ 9,000,000us-gaap_OtherInvestments       $ 10,300,000us-gaap_OtherInvestments $ 9,000,000us-gaap_OtherInvestments  
Minimum | Associated assets                      
Summary Of Significant Accounting Policies [Line Items]                      
Useful life of assets                 5 years    
Minimum | Other property and equipment                      
Summary Of Significant Accounting Policies [Line Items]                      
Useful life of assets                 3 years    
Maximum | Associated assets                      
Summary Of Significant Accounting Policies [Line Items]                      
Useful life of assets                 10 years    
Maximum | Other property and equipment                      
Summary Of Significant Accounting Policies [Line Items]                      
Useful life of assets                 10 years