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Derivatives - Cash Settlements and Valuation Gains and Losses on Commodity Derivative Contracts and Interest Rate Swaps (Detail) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended 6 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Jun. 30, 2013
Jun. 30, 2012
Derivative Instruments, (Gain) Loss        
Unrealized gain on derivative contracts     $ (63,520) $ (455,138)
(Gain) loss on derivative contracts (103,654) (669,850) (62,757) (415,204)
Commodity Derivatives
        
Derivative Instruments, (Gain) Loss        
Realized (gain) loss (17,717) [1] (89,120) [1] (1,632) [1] 36,336 [1]
Unrealized gain on derivative contracts (85,937) (580,730) (61,125) (451,540)
(Gain) loss on derivative contracts (103,654) (669,850) (62,757) (415,204)
Interest Rate Swap
        
Derivative Instruments, (Gain) Loss        
Realized (gain) loss 0 2,294 2,409 4,494
Unrealized gain on derivative contracts 0 (2,245) (2,395) (3,599)
(Gain) loss on derivative contracts $ 0 $ 49 $ 14 $ 895
[1] The three-month periods ended June 30, 2013 and 2012 included $0.7 million and $57.3 million, respectively, of realized gains related to settlements of commodity derivative contracts with contractual maturities after the quarterly period in which they were settled (“early settlements”). The six-month periods ended June 30, 2013 and 2012 included $29.0 million and $(57.3) million, respectively, of realized losses (gains) related to early settlements. The six-month period ended June 30, 2012 also included $117.1 million of non-cash realized losses on derivative contracts amended in January 2012.