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Fair Value Measurements (Hierarchy Of Fair Value, Assets And Liabilities Measured On Recurring Basis) (Detail) (USD $)
In Thousands, unless otherwise specified
Jun. 30, 2012
Dec. 31, 2011
Assets:    
Total assets $ 199,033 $ 254,781
Liabilities:    
Total liabilities   9,412
Money Market Funds [Member]
    
Assets:    
Total assets 145,609 250,419
Commodity Derivative Instruments [Member]
    
Assets:    
Total assets 53,424 4,362
Liabilities:    
Total liabilities   9,412
Fair Value, Inputs, Level 1 [Member]
    
Assets:    
Total assets 145,609 250,419
Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member]
    
Assets:    
Total assets 145,609 250,419
Fair Value, Inputs, Level 2 [Member]
    
Assets:    
Total assets 53,424  
Fair Value, Inputs, Level 2 [Member] | Commodity Derivative Instruments [Member]
    
Assets:    
Total assets 53,424  
Fair Value, Inputs, Level 3 [Member]
    
Assets:    
Total assets   4,362
Liabilities:    
Total liabilities   9,412
Fair Value, Inputs, Level 3 [Member] | Commodity Derivative Instruments [Member]
    
Assets:    
Total assets   4,362
Liabilities:    
Total liabilities   $ 9,412