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Asset Retirement Obligations - Information Related to Asset Retirement Obligations (Detail) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2014
Asset Retirement Obligation Disclosure [Abstract]  
Beginning Balance $ 200,831
Additions to asset retirement obligations 584
Accretion expense 1,425
Liabilities settled (693)
Ending Balance $ 202,147