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Asset Retirement Obligation - Information Related to Asset Retirement Obligation (Detail) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Asset Retirement Obligation Disclosure [Abstract]    
Beginning Balance $ 189,659 $ 180,662
Additions to asset retirement obligations 3,741 5,434
Accretion expense 11,046 10,871
Liabilities settled (3,615) (7,308)
Ending Balance $ 200,831 $ 189,659