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Commitment and Contingencies (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Jun. 30, 2016
Sep. 30, 2015
Dec. 31, 2016
Decontamination and decommissioning obligations     $ 38.6
ARO financial assurance     29.4
Termination benefit costs $ 0.1 $ 8.7 $ 0.4