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Summary of Significant Accounting Policies - Net Changes in Capitalized Exploratory Well Costs (Detail) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Capitalized Costs Relating To Oil And Gas Properties [Roll Forward]      
Beginning of period $ 40,424 $ 20,207 $ 5,176
Exploratory well cost additions (pending determination of proved reserves) 346,814 160,813 73,947
Exploratory well cost reclassifications (successful determination of proved reserves) (264,023) (140,091) (57,646)
Exploratory well dry hole costs (unsuccessful in adding proved reserves)   (505) (1,270)
End of period $ 123,215 $ 40,424 $ 20,207