Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Earnings Per Share, Diluted, by Common Class, Including Two Class Method
Net earnings available to Class A common stockholders, basic	$ 10.0	$ 3.3	$ 4.2	$ 6.0	$ 0.0	$ 0.0	$ 0.0	$ 0.0	$ 23.5	$ 0.0	$ 0.0
Dilutive securities, effect on basic earnings per share									0.1
Net earnings available to Class A common stockholders, diluted									$ 23.6
Weighted-Average Class A common shares outstanding, basic (in shares)									39.4	0.0	0.0
Incremental common shares attributable to dilutive effect of share-based payment arrangements									0.1
Weighted-Average Class A common shares outstanding, diluted (in shares)									39.5	0.0	0.0
Earnings per Class A common share, basic (in usd per share)	$ 0.25	$ 0.08	$ 0.11	$ 0.15	$ 0	$ 0	$ 0	$ 0	$ 0.60	$ 0	$ 0
Earnings per Class A common share, diluted (in usd per share)	$ 0.26	$ 0.08	$ 0.11	$ 0.15	$ 0	$ 0	$ 0	$ 0	$ 0.60	$ 0	$ 0
Antidilutive securities excluded from computation of earnings per share, amount									0.1