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, 2019	Dec. 31, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total fair value	$ 2.0	$ 3.1
Prepaid expenses and other current assets
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Prepaid expenses and other current assets	2.1	3.4
Other accrued liabilities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other accrued liabilities	$ 0.1	$ 0.3