Goodwill and intangible assets - Schedule of Finite-Lived Intangible Assets (Details) - USD ($) $ in Millions	6 Months Ended
	Mar. 31, 2021	Sep. 30, 2020
Finite-Lived Intangible Assets [Line Items]
Cumulative translation adjustment	$ 12.3
Gross Carrying Amount	692.3	$ 668.8
Trademarks and trade names - indefinite lived	1,366.4	1,365.4
Total Other intangible assets, net	2,058.7	2,034.2
Accumulated Amortization	(156.0)	(125.2)
Net Carrying Amount	536.3	543.6
Total Other intangible assets, net	1,902.7	1,909.0
Trademarks and trade names - indefinite lived
Finite-Lived Intangible Assets [Line Items]
Trademarks and trade names - indefinite lived		1,365.4
Trade names
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	59.7	59.7
Accumulated Amortization	(15.8)	(13.9)
Net Carrying Amount	43.9	45.8
Customer Relationships
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	395.2	394.2
Accumulated Amortization	(74.1)	(60.8)
Net Carrying Amount	321.1	333.4
Patents
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	34.5	34.5
Accumulated Amortization	(12.1)	(10.8)
Net Carrying Amount	22.4	23.7
Proprietary technology
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	172.5	172.5
Accumulated Amortization	(48.6)	(37.6)
Net Carrying Amount	123.9	134.9
Proprietary Formula
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	21.9	2.4
Accumulated Amortization	(1.5)	(0.5)
Net Carrying Amount	20.4	1.9
Non-Compete
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	0.5	0.5
Accumulated Amortization	(0.5)	(0.4)
Net Carrying Amount	0.0	0.1
Vendor Relationships
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	8.0	5.0
Accumulated Amortization	(3.4)	(1.2)
Net Carrying Amount	4.6	$ 3.8
International
Finite-Lived Intangible Assets [Line Items]
Cumulative translation adjustment	$ 11.9