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Asset Retirement Obligations - Activity For Asset Retirement Obligations (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Asset Retirement Obligation Disclosure [Abstract]    
Asset Retirement Obligation, Beginning Balance $ 5.3 $ 5.0
Revisions and liabilities incurred 0.3 0.2
Accretion expense 0.4 0.4
Payments (0.2) (0.3)
Asset Retirement Obligation, Ending Balance $ 5.8 $ 5.3