Other assets - Narratives (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Deferred Costs Capitalized Prepaid And Other Assets Disclosure Abstract
Hosting arrangement service contract implementation cost capitalized	$ 18.2		$ 18.2		$ 17.4
Hosting arrangement service contract accumulated amortization	7.7		7.7		4.9
Hosting arrangement service contract, net value	10.5		10.5		$ 12.5
Hosting arrangement service amortization expense	$ 0.9	$ 0.5	$ 2.8	$ 1.5