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Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Components Of Income Tax Expense Benefit [Line Items]
Income tax effective rate	8.70%			64.80%	56.20%	70.80%
Income tax expense (benefit) impact of the tax act		$ (0.5)	$ 8.3
Minimum [Member]
Components Of Income Tax Expense Benefit [Line Items]
Decrease in unrecognized tax benefits		4.4
Maximum [Member]
Components Of Income Tax Expense Benefit [Line Items]
Decrease in unrecognized tax benefits		$ 6.6