Income Taxes -Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2018	Sep. 30, 2018	Sep. 30, 2017
Income Tax [Line Items]
Effective income tax rate	20.90%	24.30%	20.30%	(1.90%)
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			3.0
Transition tax obligation			4.9
Unrecognized tax benefits, uncertain tax positions	$ 6.4		6.4				$ 6.5
Unrecognized tax benefits, accrued interest	$ 0.5		$ 0.5				$ 0.4
Scenario, Forecast
Income Tax [Line Items]
Federal corporate tax rate					21.00%	24.50%