Acquisitions - Summary of Acquisition and Integration Related Charges Incurred (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Business Acquisition [Line Items]
Total acquisition and integration related charges	$ 21.0	$ 37.3	$ 61.1
AAG Acquisition [Member]
Business Acquisition [Line Items]
Total acquisition and integration related charges	3.2	14.6	21.8
Shaser, Inc. [Member]
Business Acquisition [Line Items]
Total acquisition and integration related charges	1.2	0.0	0.0
HHI Business
Business Acquisition [Line Items]
Total acquisition and integration related charges	5.9	13.3	12.0
Petmatrix LLC [Member]
Business Acquisition [Line Items]
Total acquisition and integration related charges	4.5	0.0	0.0
GloFish [Member]
Business Acquisition [Line Items]
Total acquisition and integration related charges	1.0	0.0	0.0
Salix Animal Health LLC [Member]
Business Acquisition [Line Items]
Total acquisition and integration related charges	0.7	2.1	10.7
European IAMS and Eukanuba [Member]
Business Acquisition [Line Items]
Total acquisition and integration related charges	0.2	3.5	9.3
Other
Business Acquisition [Line Items]
Total acquisition and integration related charges	$ 4.3	$ 3.8	$ 7.3