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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, 2012
Dec. 31, 2011
Dec. 31, 2010
Acquisitions:      
Unproved leasehold     $ 3,697
Proved oil and gas properties     130,767
Asset retirement obligations     556
Acreage purchases 188,843 220,576 151,572
Development 1,049,129 1,007,049 727,720
Exploration:      
Drilling 309,816 226,920 50,433
Expense 65,758 77,259 56,298
Stock-based compensation expense 4,049 4,108 4,209
Gas gathering facilities:      
Development 41,035 53,387 19,627
Subtotal 1,658,630 1,589,299 1,144,879
Asset retirement obligations 57,982 24,061 (6,370)
Total - continuing operations 1,716,612 1,613,360 1,138,509
Discontinued operations   3,241 73,369
Total costs incurred $ 1,716,612 $ 1,616,601 $ 1,211,878