Employee Benefit Plans - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Retirement Benefits [Abstract]
Eligible employees contribution to 401(k) plan (up to 25%)	25.00%
Employer contributions into 401(k) plans	$ 8.8	$ 8.4	$ 7.3
Employee incentive plans expenses	$ 48.4	$ 41.6	$ 40.0