Revenue (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020
Disaggregation of Revenue
Net sales	$ 1,371.5	$ 1,247.1	$ 988.3
Revenue Benchmark | Geographic Concentration Risk | Non-US
Disaggregation of Revenue
Percentage of net sales	11.30%	11.70%	11.10%
Revenue Benchmark | Geographic Concentration Risk | Europe as a percentage of Non-US
Disaggregation of Revenue
Percentage of net sales		34.10%	41.50%
Revenue Benchmark | Geographic Concentration Risk | Canada as a percentage of Non-US
Disaggregation of Revenue
Percentage of net sales	35.40%
Revenue Benchmark | Customer Concentration Risk | One customer
Disaggregation of Revenue
Percentage of net sales	31.90%	31.50%	31.60%
Revenue Benchmark | Customer Concentration Risk | The other customer
Disaggregation of Revenue
Percentage of net sales	31.60%	33.80%	35.70%
Shakes and other beverages
Disaggregation of Revenue
Net sales	$ 1,084.0	$ 1,014.2	$ 810.1
Powders
Disaggregation of Revenue
Net sales	242.2	178.6	121.7
Nutrition Bars
Disaggregation of Revenue
Net sales	36.0	45.2	49.3
Other Products
Disaggregation of Revenue
Net sales	$ 9.3	$ 9.1	$ 7.2