Debt and standby letters of credit facilities - Interest (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Debt Disclosure [Abstract]
Interest expense on debt	$ 8.1	$ 7.7	$ 23.8	$ 23.3
Total interest paid	$ 3.3	$ 2.8	$ 18.5	$ 17.7