Revenue (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue
Net sales	$ 1,247.1	$ 988.3	$ 854.4
Revenue Benchmark | Geographic Concentration Risk | Non-US
Disaggregation of Revenue
Percentage of net sales	11.70%	11.10%	13.60%
Revenue Benchmark | Geographic Concentration Risk | Europe as a percentage of Non-US
Disaggregation of Revenue
Percentage of net sales	34.10%	41.50%	38.80%
Revenue Benchmark | Customer Concentration Risk | Largest customer
Disaggregation of Revenue
Percentage of net sales	33.80%	35.70%	32.00%
Revenue Benchmark | Customer Concentration Risk | Second largest customer
Disaggregation of Revenue
Percentage of net sales	31.50%	31.60%	37.90%
Shakes and other beverages
Disaggregation of Revenue
Net sales	$ 1,014.2	$ 810.1	$ 662.1
Powders
Disaggregation of Revenue
Net sales	178.6	121.7	119.8
Nutrition Bars
Disaggregation of Revenue
Net sales	45.2	49.3	62.5
Other Products
Disaggregation of Revenue
Net sales	$ 9.1	$ 7.2	$ 10.0