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Asset Retirement Obligations - Information Related to Asset Retirement Obligations (Detail)
$ in Thousands
3 Months Ended
Mar. 31, 2016
USD ($)
Asset Retirement Obligation Disclosure [Abstract]  
Beginning Balance $ 206,234
Additions to asset retirement obligations 4,562
Accretion expense 1,069
Liabilities settled (1,605)
Ending Balance $ 210,260