Income taxes - Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Gross unrecognized tax benefits, beginning balance	$ 8.8	$ 2.0	$ 1.8
Increases for tax positions for prior years	0.5	0.0	0.0
Unrecognized tax benefits acquired in business combinations	0.0	6.5
Decreases for tax positions for prior years	0.0	0.0	0.0
Increases for tax positions for current year	1.5	0.3	0.5
Settlements	(0.4)	0.0	(0.3)
Lapse of statute of limitations	0.0	0.0	0.0
Gross unrecognized tax benefits, ending balance	$ 10.4	$ 8.8	$ 2.0