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Commitment and Contingencies (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Decontamination and decommissioning obligations $ 1.0  
ARO financial assurance - current 16.1  
Accrued lease turnover costs 0.8  
ARO financial assurance - noncurrent 13.5  
Termination benefit costs 3.5 $ 0.4
Operating Lease, Expense $ 3.1 $ 2.9