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Summary of Significant Accounting Policies - Warranty Activity (Details) - Warranty Reserves - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Movement in Extended Product Warranty Accrual [Roll Forward]    
Balance at beginning of period $ 29.9 $ 29.6
Amounts accrued 50.3 50.0
Warranties charged to accrual (43.5) (49.7)
Balance at end of period $ 36.7 $ 29.9