Stock-Based Compensation - Summary of Weighted-Average Grant-Date Fair Value and Intrinsic Value of Restricted Units Vested (Detail) - Restricted Stock Units - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average grant-date fair value per share	$ 16.52	$ 15.47	$ 16.31
Total intrinsic value of shares vested (in millions)	$ 125.2	$ 146.5	$ 146.0