Segments - Summary of Certain Financial Data For Each of Segments (Parenthetical) (Details) - USD ($) $ in Millions		9 Months Ended	12 Months Ended
		Sep. 30, 2017	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Equity-based compensation expense	[1]	$ 3.8	$ 15.7	$ 28.8
IPO
Segment Reporting Information [Line Items]
Equity-based compensation expense				$ 19.6

[1] Represents equity-based compensation expense recognized for equity incentive plans outstanding, including $19.6 related to the IPO in the fiscal year ended September 30, 2018.