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Supplemental Information on Natural Gas and Oil Exploration, Development and Production Activities (Unaudited) - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2013
Mcf
Dec. 31, 2012
Mcf
Dec. 31, 2011
Mcf
Reserve Quantities [Line Items]      
Percentage of proved resources reviewed by consultants 96.00%    
Statement That Reserve Estimates Differ From Auditors more than 10%    
Proved Reserve And Pretax Present Value of Reserve Discounted 10.00%    
Variance in reserve estimates less than 5%.    
Scheduled to be drilling period maximum for undeveloped reserves Scheduled to be drilled within five years    
Proved reserves from drilling activities and evaluations of proved areas 1,700,000,000 1,800,000,000 1,500,000,000
Reserve attributable to natural gas 49.00% 56.00% 87.00%
Positive performance revisions for ethane reserves 676,000,000 307,000,000  
Revisions of previous estimates 449,000,000 109,000,000 225,000,000
Cost spent on undeveloped reserves transferred to developed reserves $ 504.1    
Volume of Mmcfe of reserves that have been reported for five or more years included in proved undeveloped reserves, approximately 9,300,000    
Period for which the reserves have been reported to be included in proved undeveloped reserves Five or more years.    
Maximum
      
Reserve Quantities [Line Items]      
Percentage of proved undeveloped reserves 1.00%    
Year 2014
      
Reserve Quantities [Line Items]      
Estimated future development costs 755.0    
Year 2015
      
Reserve Quantities [Line Items]      
Estimated future development costs 1,300.0    
Year 2016
      
Reserve Quantities [Line Items]      
Estimated future development costs $ 1,000.0    
Crude Oil
      
Reserve Quantities [Line Items]      
Estimate reserve information average realized prices 86.66 86.91 85.59
Proved reserves from drilling activities and evaluations of proved areas 10,723 15,131 4,915
Revisions of previous estimates 2,935 2,316 6,522
NGLs
      
Reserve Quantities [Line Items]      
Estimate reserve information average realized prices 25.93 32.23 49.24
Proved reserves from drilling activities and evaluations of proved areas 135,810 113,392 26,591
Positive performance revisions for ethane reserves 155,800 51,200  
Revisions of previous estimates 7,743 3,036 18,627
Natural Gas
      
Reserve Quantities [Line Items]      
Estimate reserve information average realized prices 3.75 2.75 3.55
Proved reserves from drilling activities and evaluations of proved areas 853,746,000 996,059,000 1,304,324,000
Revisions of previous estimates 384,825,000 76,925,000 73,643,000
Crude Oil and NGL
      
Reserve Quantities [Line Items]      
Benchmark price used for calculating average realized prices 97.33 95.05 95.61
Natural Gas, Per Thousand Cubic Feet
      
Reserve Quantities [Line Items]      
Benchmark price used for calculating average realized prices 3.67 2.76 4.12