Revenue (Details) - USD ($) $ 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
Disaggregation of Revenue
Net sales	$ 282.6	$ 204.2	$ 257.5	$ 244.0	$ 214.5	$ 237.6	$ 216.5	$ 185.8	$ 988.3	$ 854.4	$ 827.5
Largest customer
Disaggregation of Revenue
Percentage of net sales									35.70%	32.00%	34.50%
Second largest customer
Disaggregation of Revenue
Percentage of net sales									31.60%	37.90%	36.10%
Non-US
Disaggregation of Revenue
Percentage of net sales									11.10%	13.60%	14.50%
Europe as a percentage of Non-US
Disaggregation of Revenue
Percentage of net sales									41.50%	38.80%	43.00%
Shakes and other beverages
Disaggregation of Revenue
Net sales									$ 810.1	$ 662.1	$ 608.5
Powders
Disaggregation of Revenue
Net sales									121.7	119.8	114.9
Nutrition Bars
Disaggregation of Revenue
Net sales									49.3	62.5	92.5
Other Products
Disaggregation of Revenue
Net sales									$ 7.2	$ 10.0	$ 11.6