Capitalized contract costs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Capitalized Contract Cost [Line Items]
Costs to fulfill a contract - amortization expense	$ 24.2	$ 21.7
Costs to fulfill a contract	75.9	79.5
Deferred Commissions
Capitalized Contract Cost [Line Items]
Deferred commissions	12.4	12.1
Costs to fulfill a contract - amortization expense	$ 3.8	$ 6.9