Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ 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
Quarterly Financial Information Disclosure [Abstract]
Net sales	$ 282.6	$ 204.2	$ 257.5	$ 244.0	$ 214.5	$ 237.6	$ 216.5	$ 185.8	$ 988.3	$ 854.4	$ 827.5
Gross profit	89.8	68.7	88.2	91.3	76.7	90.5	79.0	65.6	338.0	311.8	277.7
Net earnings including redeemable noncontrolling interest	35.5	14.2	18.6	31.8	26.7	40.3	31.0	25.1	100.1	123.1	96.1
Net earnings available to Class A common stockholders	$ 10.0	$ 3.3	$ 4.2	$ 6.0	$ 0.0	$ 0.0	$ 0.0	$ 0.0	$ 23.5	$ 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