Restructuring and related charges - Summarization of Restructuring and Related Charges (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 62.5	$ 15.2	$ 28.7
Consumer products cost of goods sold classification
Restructuring Cost and Reserve [Line Items]
Restructuring charges	18.3	0.5	2.1
Selling, acquisition, operating and general expenses classification
Restructuring Cost and Reserve [Line Items]
Restructuring charges	44.2	14.7	26.6
Global Cost Reduction Initiatives
Restructuring Cost and Reserve [Line Items]
Restructuring charges	24.2	5.3	0.0
HHI Business Rationalization Initiatives [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges	0.0	$ 1.8	10.3
Expected Completion Date		Sep. 30, 2016
Global Expense Rationalization Initiatives
Restructuring Cost and Reserve [Line Items]
Restructuring charges	0.0	$ 5.2	17.1
Total Projected Costs	32.0
Expected Completion Date		Sep. 30, 2016
HHI Distribution Center Consolidation [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges	27.4	$ 0.0	0.0
Total Projected Costs	50.0
Pet Rightsizing Initiative [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges	8.2	0.0	0.0
Total Projected Costs	11.0
Other initiatives
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 2.7	$ 2.9	$ 1.3