Employee benefit plans - Amounts recognized and significant components (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Amount recognized in the balance sheet
Non-current liabilities	$ 3.1	$ 1.2
Amounts recognized in accumulated OCL
Unrecognized loss	(3.1)	(1.7)
Balances of significant components of the pension plan
Projected benefit obligation	17.4	14.1	$ 13.2
Accumulated benefit obligation	16.4	12.9
Fair value of plan assets	14.3	12.9	$ 12.0
Amount invested in common collective trust	14.3	$ 12.9
Amount of investments categorized as Level 1 or Level 3	$ 0.0