Income taxes - Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Gross unrecognized tax benefits, beginning of period	$ 9.8	$ 9.2	$ 10.4
Decreases for tax positions for prior years	(1.5)
Increases (decreases) for tax positions for prior years		0.4	0.0
Increases for tax positions for current year	0.9	0.2	0.6
Settlements	0.0	0.0	(1.8)
Lapse of statute of limitations	(8.0)	0.0	0.0
Gross unrecognized tax benefits, end of period	$ 1.2	$ 9.8	$ 9.2