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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
Mar. 31, 2013
Mar. 31, 2012
Derivative Instruments, (Gain) Loss [Line Items]    
Unrealized losses (gains) on derivatives $ 22,417 $ 127,836
Commodity Derivatives
   
Derivative Instruments, (Gain) Loss [Line Items]    
Realized loss 16,085 [1] 125,456 [1]
Unrealized losses (gains) on derivatives 24,812 129,190
Loss on derivative contracts 40,897 254,646
Interest rate swap
   
Derivative Instruments, (Gain) Loss [Line Items]    
Realized loss 2,409 2,200
Unrealized losses (gains) on derivatives (2,395) (1,354)
Loss on derivative contracts $ 14 $ 846
[1] The three-month period ended March 31, 2013 includes $29.6 million of realized losses related to settlements of commodity derivative contracts with contractual maturities after the quarterly period in which they were settled (“early settlements”) in conjunction with the sale of the Permian Properties. The three-month period ended March 31, 2012 includes $117.1 million of non-cash realized losses on derivative contracts amended in January 2012.