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Stock-Based Compensation Plans - Allocation of Stock-Based Compensation by Functional Category (Detail) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]      
Total $ 64,369 $ 52,770 $ 43,794
Operating Expense
      
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]      
Total 2,755 2,415 1,987
Brokered Natural Gas and Marketing Expense
      
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]      
Total 1,852 1,765 1,455
Exploration Expense
      
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]      
Total 4,025 4,049 4,108
General and Administrative Expense
      
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]      
Total $ 55,737 $ 44,541 $ 36,244