Capitalized contract costs - Summary (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Capitalized Contract Cost [Line Items]
Costs to fulfill a contract - amortization expense	$ 27.9	$ 27.5	$ 24.2
Costs to fulfill a contract	56.2	74.4
Deferred Commissions
Capitalized Contract Cost [Line Items]
Deferred commissions	6.7	8.7
Costs to fulfill a contract - amortization expense	$ 2.9	$ 3.2	$ 3.1