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Asset Retirement Obligations - Activity For Asset Retirement Obligations (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Asset Retirement Obligation Disclosure [Abstract]    
Balance, beginning of period $ 5.0  
Obligations assumed in business combination   $ 6.5
Revisions and liabilities incurred 0.2 (1.0)
Accretion expense 0.4 0.4
Payments (0.3) (0.9)
Balance, end of period $ 5.3 $ 5.0