Goodwill and intangible assets (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Finite-Lived Intangible Assets [Line Items]
Indefinite-live intangible assets	$ 1,363.8	$ 76.9
Cumulative translation adjustment	$ (4.6)	(0.5)
Remaining life (in years)	10 years 3 months 18 days
Amortization of intangible assets	$ 43.2	$ 11.5	$ 11.2
Amortization expense next year	55.3
Amortization expense year two	55.2
Amortization expense year three	55.2
Amortization expense year four	51.8
Amortization expense year five	50.7
Amortization expense thereafter	$ 326.9
Minimum
Finite-Lived Intangible Assets [Line Items]
Amortization period, years	4 years
Maximum
Finite-Lived Intangible Assets [Line Items]
Amortization period, years	15 years
Battery Acquisition
Finite-Lived Intangible Assets [Line Items]
Increase in goodwill	$ 587.0
Spectrum Auto Care Acquisition
Finite-Lived Intangible Assets [Line Items]
Increase in goodwill	701.6
Cumulative translation adjustment	$ (1.7)