Restructuring and related charges - Summary of restructuring and related charges by type and classification (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Cost, Incurred Cost	$ 62.5	$ 15.2	$ 28.7
Restructuring and Related Cost, Cost Incurred to Date	67.8
Restructuring and Related Cost, Expected Cost Remaining	31.0
Global Expense Rationalization Initiatives
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Cost, Incurred Cost	0.0	5.2	17.1
Restructuring and Related Cost, Cost Incurred to Date	29.5	47.0
Global Cost Reduction Initiatives
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Cost, Incurred Cost	24.2	5.3	0.0
Special Termination Benefits [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Cost, Incurred Cost	12.8	4.3	7.0
Restructuring and Related Cost, Cost Incurred to Date	13.1
Restructuring and Related Cost, Expected Cost Remaining	6.0
Other Restructuring [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Cost, Incurred Cost	49.7	$ 10.9	$ 21.7
Restructuring and Related Cost, Cost Incurred to Date	54.7
Restructuring and Related Cost, Expected Cost Remaining	$ 25.0