Restructuring and related charges - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Cost, Cost Incurred to Date	$ 109.1		$ 109.1
Restructuring charges	23.2	$ 8.0	43.6	$ 10.2
Selling, acquisition, operating and general expenses classification
Restructuring Cost and Reserve [Line Items]
Restructuring charges	23.2	8.0	43.6	10.2
Pet Rightsizing Initiative [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Cost, Cost Incurred to Date	12.2		12.2
Restructuring and Related Cost, Expected Cost	13.0		13.0
Restructuring charges	3.4	0.6	4.0	0.6
HHI Distribution Center Consolidation [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Cost, Cost Incurred to Date	56.2		56.2
Restructuring and Related Cost, Expected Cost	67.0		67.0
Restructuring charges	13.6	$ 1.2	7.1	$ 7.0
Global Expense Rationalization Initiatives
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Cost, Cost Incurred to Date	36.7		36.7
Restructuring and Related Cost, Expected Cost	$ 41.0		$ 41.0