Annual report pursuant to Section 13 and 15(d)

Percentage of Revenue by Major Customers (Detail)

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Percentage of Revenue by Major Customers (Detail)
 12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Entity Wide Revenue Major Customer [Line Items]      
Percentage of receipts 10.00%    
Shell Trading (US) Co.
      
Entity Wide Revenue Major Customer [Line Items]      
Percentage of receipts 48.00% 35.00% 36.00%
ConocoPhillips
      
Entity Wide Revenue Major Customer [Line Items]      
Percentage of receipts    [1],[2] 16.00% [1] 16.00% [1]
J.P. Morgan Ventures Energy Corp.
      
Entity Wide Revenue Major Customer [Line Items]      
Percentage of receipts    [2]    [2] 10.00%
[1] ConocoPhillips split into two separate companies during 2012 and individually were approximately 8% each.
[2] less than 10%