Income taxes - Reconciliation of unrecognized tax benefits (Details) - USD ($) $ in Millions	3 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018
Income Tax Disclosure
Beginning Balance	$ 7.7	$ 7.5	$ 7.2	$ 7.8	$ 7.5	$ 7.3
Addition for tax positions	0.0	0.2	0.3	0.3	0.3	0.2
Reduction as a result of lapse of statute of limitations	0.0			(1.2)
Ending Balance	$ 7.7	$ 7.7	$ 7.5	$ 6.9	$ 7.8	$ 7.5