Income Taxes -Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Sep. 30, 2018	Sep. 30, 2017
Income Tax [Line Items]
Effective income tax rate	19.60%	(32.30%)
Provisional charges for the re-measurement of deferred tax assets, long-term	$ 5.6
Provisional charges for the re-measurement of deferred tax assets, short-term	1.3
Transition tax obligation	4.9
Unrecognized tax benefits, uncertain tax positions	6.1				$ 6.5
Unrecognized tax benefits, accrued interest	$ 0.4				$ 0.4
Scenario, Forecast
Income Tax [Line Items]
Federal corporate tax rate			21.00%	24.50%