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Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2012
Dec. 31, 2011
Assets:    
Commodity derivative assets - current $ 11,292 $ 18,987
Commodity derivative assets - noncurrent 5,181 0
Liabilities:    
Commodity derivative liabilities - current 0 (3,549)
Commodity derivative liabilities - noncurrent (1,421) (6,850)
Fair Value, Measurements, Recurring
    
Assets:    
Assets of SERP 3,237 3,088
Liabilities:    
Deferred compensation - SERP obligation and other (3,305) (3,168)
Fair Value, Measurements, Recurring | Level 1
    
Assets:    
Assets of SERP 3,237 3,088
Liabilities:    
Deferred compensation - SERP obligation and other (3,305) (3,168)
Fair Value, Measurements, Recurring | Level 2
    
Assets:    
Assets of SERP 0 0
Liabilities:    
Deferred compensation - SERP obligation and other 0 0
Fair Value, Measurements, Recurring | Level 3
    
Assets:    
Assets of SERP 0 0
Liabilities:    
Deferred compensation - SERP obligation and other 0 0
Fair Value, Measurements, Recurring | Commodity contracts
    
Assets:    
Commodity derivative assets - current 11,292 18,987
Commodity derivative assets - noncurrent 5,181  
Liabilities:    
Commodity derivative liabilities - current 0 (3,549)
Commodity derivative liabilities - noncurrent (1,421) (6,850)
Fair Value, Measurements, Recurring | Commodity contracts | Level 1
    
Assets:    
Commodity derivative assets - current 0 0
Commodity derivative assets - noncurrent 0  
Liabilities:    
Commodity derivative liabilities - current 0 0
Commodity derivative liabilities - noncurrent 0 0
Fair Value, Measurements, Recurring | Commodity contracts | Level 2
    
Assets:    
Commodity derivative assets - current 11,292 18,987
Commodity derivative assets - noncurrent 5,181  
Liabilities:    
Commodity derivative liabilities - current 0 (3,549)
Commodity derivative liabilities - noncurrent (1,421) (6,850)
Fair Value, Measurements, Recurring | Commodity contracts | Level 3
    
Assets:    
Commodity derivative assets - current 0 0
Commodity derivative assets - noncurrent 0  
Liabilities:    
Commodity derivative liabilities - current 0 0
Commodity derivative liabilities - noncurrent $ 0 $ 0