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Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2013
Dec. 31, 2012
Assets:    
Commodity derivative assets - current $ 5,900 $ 11,292
Commodity derivative assets - noncurrent 3,608 5,181
Liabilities:    
Commodity derivative liabilities - current (4,539) 0
Commodity derivative liabilities - noncurrent (1,235) (1,421)
Fair Value, Measurements, Recurring | Fair Value Measurement
    
Assets:    
Assets of SERP 3,401 3,237
Liabilities:    
Deferred compensation - SERP obligation (3,464) (3,305)
Fair Value, Measurements, Recurring | Level 1
    
Assets:    
Assets of SERP 3,401 3,237
Liabilities:    
Deferred compensation - SERP obligation (3,464) (3,305)
Fair Value, Measurements, Recurring | Level 2
    
Assets:    
Assets of SERP 0 0
Liabilities:    
Deferred compensation - SERP obligation 0 0
Fair Value, Measurements, Recurring | Level 3
    
Assets:    
Assets of SERP 0 0
Liabilities:    
Deferred compensation - SERP obligation 0 0
Fair Value, Measurements, Recurring | Commodity contracts | Fair Value Measurement
    
Assets:    
Commodity derivative assets - current 5,900 11,292
Commodity derivative assets - noncurrent 3,608 5,181
Liabilities:    
Commodity derivative liabilities - current (4,539)  
Commodity derivative liabilities - noncurrent (1,235) (1,421)
Fair Value, Measurements, Recurring | Commodity contracts | Level 1
    
Assets:    
Commodity derivative assets - current 0 0
Commodity derivative assets - noncurrent 0 0
Liabilities:    
Commodity derivative liabilities - current 0  
Commodity derivative liabilities - noncurrent 0 0
Fair Value, Measurements, Recurring | Commodity contracts | Level 2
    
Assets:    
Commodity derivative assets - current 5,900 11,292
Commodity derivative assets - noncurrent 3,608 5,181
Liabilities:    
Commodity derivative liabilities - current (4,539)  
Commodity derivative liabilities - noncurrent (1,235) (1,421)
Fair Value, Measurements, Recurring | Commodity contracts | Level 3
    
Assets:    
Commodity derivative assets - current 0 0
Commodity derivative assets - noncurrent 0 0
Liabilities:    
Commodity derivative liabilities - current 0  
Commodity derivative liabilities - noncurrent $ 0 $ 0