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Fair Value Measurements - Additional Information (Detail) (USD $)
Jun. 30, 2013
Dec. 31, 2012
Fair Value Disclosures [Abstract]    
Derivative Credit Risk Valuation Adjustment, Derivative Assets $ 76,000 $ 29,000
Long-term debt 1,200,000,000 1,200,000,000
Long-term debt, fair value 1,239,000,000  
Asset retirement obligation, carrying value $ 26,700,000 $ 23,234,000