Restructuring and related charges - Summary of restructuring and related charges by type and classification (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Cost, Incurred Cost	$ 15.2	$ 28.7	$ 22.9
Restructuring and Related Cost, Cost Incurred to Date	71.8
Global Expense Rationalization Initiatives
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Cost, Incurred Cost	5.3	0.0	0.0
Restructuring and Related Cost, Cost Incurred to Date	47.0
Global Cost Reduction Initiatives
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Cost, Incurred Cost	5.2	17.1	13.4
Restructuring and Related Cost, Cost Incurred to Date	5.3
Special Termination Benefits [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Cost, Incurred Cost	4.3	7.0	11.2
Restructuring and Related Cost, Cost Incurred to Date	32.4
Other Restructuring [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Cost, Incurred Cost	10.9	$ 21.7	$ 11.7
Restructuring and Related Cost, Cost Incurred to Date	$ 39.4