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Summary of Significant Accounting Policies - Discontinued Operations (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Disposal Groups        
Loss from discontinued operations, net of income taxes   $ 0.0 $ 0.8 $ 0.0
Lahnstein Mill        
Disposal Groups        
Loss from discontinued operations, net of income taxes $ 0.8