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Supplemental Information on Natural Gas and Oil Exploration, Development and Production Activities (Unaudited) - Capitalized Costs and Accumulated Depreciation, Depletion and Amortization (Detail) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Natural gas and oil properties:      
Properties subject to depletion $ 8,225,859 [1] $ 7,368,308 [1] $ 6,035,429 [1]
Unproved properties 807,022 [1] 743,467 [1] 748,598 [1]
Total 9,032,881 [1] 8,111,775 [1] 6,784,027 [1]
Accumulated depreciation, depletion and amortization (2,274,444) [1] (2,015,591) [1] (1,626,461) [1]
Net capitalized costs $ 6,758,437 [1] $ 6,096,184 [1] $ 5,157,566 [1]
[1] Includes capitalized asset retirement costs and the associated accumulated amortization.