Goodwill and intangible assets - Schedule of Finite-Lived Intangible Assets (Details) - USD ($) $ in Millions	9 Months Ended
	Jun. 30, 2020	Sep. 30, 2019
Finite-Lived Intangible Assets [Line Items]
Cumulative translation adjustment	$ (0.9)
Gross Carrying Amount	669.5	$ 663.8
Trademarks and trade names - indefinite lived	1,364.9	1,363.8
Total Other intangible assets, net	2,034.4	2,027.6
Accumulated Amortization	(111.0)	(68.7)
Net Carrying Amount	558.5	595.1
Total Other intangible assets, net	1,923.4	1,958.9
Trade names
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	59.7	59.7
Accumulated Amortization	(12.9)	(9.9)
Net Carrying Amount	46.8	49.8
Customer Relationships
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	394.2	394.2
Accumulated Amortization	(54.2)	(34.3)
Net Carrying Amount	340.0	359.9
Patents
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	34.5	34.5
Accumulated Amortization	(10.2)	(8.2)
Net Carrying Amount	24.3	26.3
Proprietary technology
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	172.5	172.5
Accumulated Amortization	(32.1)	(15.7)
Net Carrying Amount	140.4	156.8
Proprietary Formula
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	2.4	2.4
Accumulated Amortization	(0.4)	(0.3)
Net Carrying Amount	2.0	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	5.7
Accumulated Amortization	(0.8)
Net Carrying Amount	4.9
International
Finite-Lived Intangible Assets [Line Items]
Cumulative translation adjustment	$ (0.6)