Financial instruments and concentration of credit risks - Fair Value of Foreign Exchange Forward Contracts by Balance Sheet Location (Details) - Foreign Exchange Forward Contract - Level 2 - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total fair value	$ 3.1	$ 3.3
Prepaid expenses and other current assets
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative asset	3.4	4.9
Other accrued liabilities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liability	$ 0.3	$ 1.6