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Fair Value Measurements - Additional Information (Detail) (USD $)
Sep. 30, 2013
Dec. 31, 2012
Debt Instrument [Line Items]    
Derivative credit risk valuation adjustment, derivative assets $ 400,000 $ 29,000
Long-term debt 2,360,000,000 1,200,000,000
Asset retirement obligation, carrying value 27,431,000 23,234,000
Senior Unsecured Debt [Member]
   
Debt Instrument [Line Items]    
Long-term debt 2,200,000,000  
Long-term debt, fair value 2,325,000,000  
Second Amended Credit Facility [Member]
   
Debt Instrument [Line Items]    
Long-term debt $ 160,000,000