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Income Taxes - Schedule Of Gross Unrecognized Income Tax Benefits (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Reconciliation of Unrecognized Tax Benefits [Roll Forward]      
Beginning balance $ 106.0 $ 92.4 $ 75.5
Increases related to tax positions taken during the current year 21.1 29.9 28.9
Increases related to tax positions taken during a prior year 0.0 0.0 0.3
Decreases related to tax positions taken during a prior year (46.5) (0.5) 0.0
Decreases related to settlements with tax authorities (0.5) 0.0 (11.4)
Decreases related to expiration of statute of limitations (14.7) (15.8) (0.9)
Ending balance $ 65.4 $ 106.0 $ 92.4