Income tax expense - Other (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Reconciliation of income taxes
Tax expense	$ (24.2)	$ 9.8
Effective Income Tax Rate Reconciliation at Federal Statutory Income Tax Rate, Amount	13.3	(9.2)
Impact related to change in estimates	3.0
Other permanent differences	0.3	0.4
Net deferred tax liability		(13.3)
Net deferred tax liability	17.2
Valuation allowance	105.2	145.4
Increase (decrease) in the valuation allowance	(40.2)	5.7
Foreign exchange	0.6
Dividend withholding tax and other cash taxes		1.3
Impairments		7.7
US
Reconciliation of income taxes
Tax expense	(25.3)	9.6
Effective Income Tax Rate Reconciliation at Federal Statutory Income Tax Rate, Amount	11.2	(2.2)
Net deferred tax liability		(23.7)
Net deferred tax liability	4.5
Capital gain on intercompany notes		0.1
Changes in tax rates	(0.1)	2.2
Increase (decrease) in the valuation allowance	(39.7)	(2.2)
Operating in countries with different income tax rates	(0.3)	0.1
Dividend withholding tax and other cash taxes	$ 1.8	1.1
Impairments		$ 7.7
Cumulative pre tax period	3 years
Canada.
Reconciliation of income taxes
Enacted statutory income tax rate	27.00%	27.00%
Tax expense	$ 1.1	$ 0.2
Effective Income Tax Rate Reconciliation at Federal Statutory Income Tax Rate, Amount	2.1	(7.0)
Net deferred tax liability	12.7	10.4
Capital gain on intercompany notes	0.2
Increase (decrease) in the valuation allowance	(0.5)	7.9
Foreign exchange	(0.6)	1.7
Dividend withholding tax and other cash taxes	$ 0.2	$ 0.2