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Basis and Summary of Significant Accounting Policies - Income Taxes (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]    
Tax Cuts And Jobs Act Of 2017, Income tax expense (benefit) $ 0.2 $ (140.0)