Debt and standby letters of credit facilities - Interest (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Debt Disclosure [Abstract]
Interest expense on debt	$ 7.9	$ 8.0	$ 15.7	$ 15.6
Total interest paid	$ 12.4	$ 12.2	$ 15.2	$ 14.9