Restructuring and related charges - Summarization of Restructuring and Related Charges (Detail) - 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 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
Global Cost Reduction Initiatives
Restructuring Cost and Reserve [Line Items]
Restructuring charges	3.1	5.5	28.8	1.2
Global Expense Rationalization Initiatives
Restructuring Cost and Reserve [Line Items]
Total Projected Costs	41.0		41.0
HHI Distribution Center Consolidation [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges	13.6	1.2	7.1	7.0
Total Projected Costs	67.0		67.0
Pet Rightsizing Initiative [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges	3.4	0.6	4.0	0.6
Total Projected Costs	13.0		13.0
Other initiatives
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 3.1	$ 0.7	$ 3.7	$ 1.4