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Supplemental Information on Natural Gas and Oil Exploration, Development and Production Activities (Unaudited) - Costs Incurred for Property Acquisition, Exploration and Development (Detail) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Costs Incurred Oil And Gas Property Acquisition Exploration And Development Activities [Line Items]      
Acreage purchases $ 137,538 [1] $ 188,843 [1] $ 220,576 [1]
Development 938,668 [1] 1,049,129 [1] 1,007,049 [1]
Gas gathering facilities:      
Development 47,086 [1] 41,035 [1] 53,387 [1]
Subtotal 1,377,443 [1] 1,658,630 [1] 1,589,299 [1]
Asset retirement costs capitalized, net 76,373 [1] 57,982 [1] 24,061 [1]
Total costs incurred 1,453,816 [1] 1,716,612 [1] 1,616,601 [1]
Drilling
      
Exploration:      
Exploration costs incurred 189,742 [1] 309,816 [1] 226,920 [1]
Expense
      
Exploration:      
Exploration costs incurred 60,384 [1] 65,758 [1] 77,259 [1]
Stock-based compensation expense
      
Exploration:      
Exploration costs incurred 4,025 [1] 4,049 [1] 4,108 [1]
Continuing Operations
      
Gas gathering facilities:      
Total costs incurred 1,453,816 [1] 1,716,612 [1] 1,613,360 [1]
Discontinued Operations
      
Gas gathering facilities:      
Total costs incurred     $ 3,241 [1]
[1] Includes cost incurred whether capitalized or expensed.