Restructuring and related charges - Summary of restructuring and related charges by type and classification (Details) - 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 and Related Cost, Incurred Cost	$ 23.2	$ 8.0	$ 43.6	$ 10.2
Restructuring and Related Cost, Cost Incurred to Date	109.1		109.1
Restructuring and Related Cost, Expected Cost Remaining	19.2		19.2
Global Expense Rationalization Initiatives
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Cost, Cost Incurred to Date	36.7		36.7
Global Cost Reduction Initiatives
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Cost, Incurred Cost	3.1	5.5	28.8	1.2
Special Termination Benefits [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Cost, Incurred Cost	4.0	1.3	5.1	2.1
Restructuring and Related Cost, Cost Incurred to Date	16.1		16.1
Restructuring and Related Cost, Expected Cost Remaining	0.4		0.4
Other Restructuring [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Cost, Incurred Cost	19.2	$ 6.7	38.5	$ 8.1
Restructuring and Related Cost, Cost Incurred to Date	93.0		93.0
Restructuring and Related Cost, Expected Cost Remaining	$ 18.8		$ 18.8