Income Taxes -Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2018	Sep. 30, 2018	Sep. 30, 2017
Income Tax [Line Items]
Effective income tax rate	22.50%	28.60%	21.10%	9.20%
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			4.4
Transition tax obligation			4.9
Unrecognized tax benefits, uncertain tax positions	$ 5.2		5.2				$ 6.5
Unrecognized tax benefits, accrued interest	$ 0.4		$ 0.4				$ 0.4
Scenario, Forecast
Income Tax [Line Items]
Federal corporate tax rate					21.00%	24.50%