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Supplemental Information on Natural Gas and Oil Exploration, Development and Production Activities (Unaudited) - Changes in Discounted Future Net Cash Flows (Detail) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Revisions of previous estimates      
Changes in prices $ 2,172,704 $ (2,498,616) $ 422,080
Revisions in quantities 513,168 88,190 326,240
Changes in future development costs (275,468) (354,766) (346,378)
Accretion of discount 395,989 608,381 464,735
Net change in income taxes (1,299,227) 832,830 (400,690)
Purchases of reserves in place         
Additions to proved reserves from extensions, discoveries and improved recovery 1,981,054 1,429,340 2,169,706
Production (1,286,103) (976,224) (911,873)
Development costs incurred during the period 462,862 562,329 513,551
Sales of natural gas and oil (162,463) (120,637) (1,313,401)
Timing and other 135,910 (861,919) 111,801
Net change for the year 2,638,426 (1,291,092) 1,035,771
Beginning of year 3,223,805 4,514,897 3,479,126
End of year $ 5,862,231 $ 3,223,805 $ 4,514,897