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Fair Value Measurements - Additional Information (Detail) (USD $)
Mar. 31, 2014
Dec. 31, 2013
Debt Instrument [Line Items]    
Derivative credit risk valuation adjustment, derivative assets $ 300,000 $ 200,000
Long-term debt 2,260,000,000 2,535,570,000
Asset retirement obligation, carrying value 36,330,000 36,458,000
Senior Unsecured Debt [Member]
   
Debt Instrument [Line Items]    
Long-term debt 2,200,000,000  
Long-term debt, fair value 2,374,500,000  
Second Amended Credit Facility [Member]
   
Debt Instrument [Line Items]    
Long-term debt $ 60,000,000