Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Earnings Per Share, Diluted, by Common Class, Including Two Class Method
Net earnings available to Class A common stockholders, basic	$ 3.3	$ 0.0	$ 13.5	$ 0.0
Dilutive securities, effect on basic earnings per share	0.0		0.0
Net Income (Loss) Attributable to Parent, Diluted, Total	$ 3.3		$ 13.5
Weighted-Average Class A common shares outstanding, basic (in shares)	39.4	0.0	39.4	0.0
Incremental common shares attributable to dilutive effect of share-based payment arrangements	0.1		0.1
Weighted-Average Class A common shares outstanding, diluted (in shares)	39.5	0.0	39.5	0.0
Earnings per Class A common share, basic (in usd per share)	$ 0.08	$ 0	$ 0.34	$ 0
Earnings per Class A common share, diluted (in usd per share)	$ 0.08	$ 0	$ 0.34	$ 0
Common Class A
Earnings Per Share, Diluted, by Common Class, Including Two Class Method
Antidilutive securities excluded from computation of earnings per share, amount	0.1		0.1