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Fair Value Measurements - Additional Information (Detail) (USD $)
Jun. 30, 2014
Dec. 31, 2013
Debt Instrument [Line Items]    
Derivative credit risk valuation adjustment, derivative assets $ 2,500,000 $ 200,000
Long-term debt 2,300,000,000 2,535,570,000
Asset retirement obligation, carrying value 38,082,000 36,458,000
Senior Unsecured Debt [Member]
   
Debt Instrument [Line Items]    
Long-term debt 2,200,000,000  
Long-term debt, fair value 2,380,000,000  
Second Amended Credit Facility [Member]
   
Debt Instrument [Line Items]    
Long-term debt $ 100,000,000