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Income Taxes - Reconciliation of Unrecognized Tax Benefits (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]      
Balance of gross unrecognized tax benefits at the beginning of the fiscal year $ 25.6 $ 15.7 $ 7.9
Gross increases related to prior period tax positions 1.1 0.0 2.2
Gross increases related to current period tax positions 33.1 9.9 5.6
Balance of gross unrecognized tax benefits at the end of the fiscal year $ 59.8 $ 25.6 $ 15.7