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Liabilities - Other Liabilities (Details) - USD ($)
$ in Millions
6 Months Ended 12 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Other Liabilities    
Asset retirement obligations $ 3.3 $ 3.3
Medical and dental benefits 2.1 2.2
Other Assets    
Other Liabilities    
Cash surrender value of life insurance policies 3.5 3.4
Other Liabilities    
Other Liabilities    
Plan liabilities $ 4.0 $ 4.7