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Income Taxes - Summary of Activities Related to Unrecognized Tax Benefits (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]      
Balance at January 1 $ 1.8 $ 2.2 $ 2.0
Increases related to current year tax 0.2 0.1 0.3
Decreases due to lapse of statute of limitations (0.1) (0.5) (0.1)
Balance at December 31 $ 1.9 $ 1.8 $ 2.2