Financial instruments and concentration of credit risks - Gains and Losses Recognized on Foreign Exchange Forward Contracts (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other income (expense), net
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Amount of gain (loss) recognized	$ 27.5	$ (29.1)	$ 15.6

Aggregate impact of foreign currency derivatives on earnings for the period. Includes impact of foreign currency cash flow hedge ineffectiveness and the ineffectiveness of fair value hedging derivatives and full change in fair value of the foreign currency derivatives not designated as hedging instruments.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624171-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5618551-113959

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4D
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624177-113959

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration