Income tax expense - Current and Deferred tax benefit (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Components of income tax expenses (benefit)
Income tax (benefit) expense	$ (24.2)	$ 9.8
US
Components of income tax expenses (benefit)
Current income tax expense	2.8	1.9
Deferred income tax (benefit) expense	(28.1)	7.7
Income tax (benefit) expense	(25.3)	9.6
Canada.
Components of income tax expenses (benefit)
Current income tax expense	2.8	3.0
Deferred income tax (benefit) expense	(1.7)	(2.8)
Income tax (benefit) expense	$ 1.1	$ 0.2