Stock-Based Compensation - Summary of Weighted-Average Grant-Date Fair Value and Intrinsic Value of Restricted Units Vested (Details) - Restricted Stock Units - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average grant-date fair value per share (in dollars per share)	$ 55.18	$ 37.26
Total intrinsic value of shares vested (in millions)	$ 213.4	$ 119.4