Income taxes - Reconciliation of unrecognized tax benefits (Details) - USD ($) $ in Millions	3 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020
Income Tax Disclosure
Beginning Balance	$ 14.8	$ 14.8	$ 14.8	$ 16.3	$ 16.3	$ 16.3
Reduction as a result of lapse of statute of limitations	(11.3)	0.0	0.0	(1.5)	0.0	0.0
Ending Balance	$ 3.5	$ 14.8	$ 14.8	$ 14.8	$ 16.3	$ 16.3