Goodwill and intangible assets (Schedule of Finite-Lived Intangible Assets) (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Finite-Lived Intangible Assets [Line Items]
Remaining life (in years)	11 years
Gross Carrying Amount	$ 668.8	$ 663.8
Accumulated Amortization	(125.2)	(68.7)
Net Carrying Amount	543.6	595.1
Trademarks and trade names - indefinite lived	1,365.4	1,363.8
Gross Carrying Amount	2,034.2	2,027.6
Net Carrying Amount	1,909.0	1,958.9
Trademarks
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	59.7	59.7
Accumulated Amortization	(13.9)	(9.9)
Net Carrying Amount	45.8	49.8
Customer relationships
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	394.2	394.2
Accumulated Amortization	(60.8)	(34.3)
Net Carrying Amount	333.4	359.9
Patents
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	34.5	34.5
Accumulated Amortization	(10.8)	(8.2)
Net Carrying Amount	23.7	26.3
Proprietary technology
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	172.5	172.5
Accumulated Amortization	(37.6)	(15.7)
Net Carrying Amount	134.9	156.8
Proprietary formulas
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	2.4	2.4
Accumulated Amortization	(0.5)	(0.3)
Net Carrying Amount	1.9	2.1
Non-Compete
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	0.5	0.5
Accumulated Amortization	(0.4)	(0.3)
Net Carrying Amount	0.1	$ 0.2
Vendor relationships
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	5.0
Accumulated Amortization	(1.2)
Net Carrying Amount	$ 3.8