Restructuring and related charges - Summary of restructuring and related charges by type and classification (Details) - USD ($) $ in Millions	3 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Cost, Incurred Cost	$ 20.4	$ 2.2
Restructuring and Related Cost, Cost Incurred to Date	86.0
Restructuring and Related Cost, Expected Cost Remaining	16.2
Global Expense Rationalization Initiatives
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Cost, Cost Incurred to Date	33.6
Global Cost Reduction Initiatives
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Cost, Incurred Cost	4.0	1.5
Special Termination Benefits [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Cost, Incurred Cost	1.1	0.8
Restructuring and Related Cost, Cost Incurred to Date	12.1
Restructuring and Related Cost, Expected Cost Remaining	0.2
Other Restructuring [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Cost, Incurred Cost	19.3	$ 1.4
Restructuring and Related Cost, Cost Incurred to Date	73.9
Restructuring and Related Cost, Expected Cost Remaining	$ 16.0

Amount of costs incurred to date for the specified restructuring cost.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 420
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6394359&loc=d3e17939-110869

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 420
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SAB Topic 5.P.4(b))
-URI http://asc.fasb.org/extlink&oid=27011515&loc=d3e140904-122747

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Amount of expected cost remaining for the specified restructuring cost.

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Discloses the amount charged against the accrued restructuring reserves, or earnings if not previously accrued, during the period for the specified type of restructuring cost.

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