Income tax expense - Reconciliation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Reconciliation of income taxes
Computed income tax expense (benefit) at Canadian statutory rate	$ 13.3	$ (9.2)
Change in valuation allowance	(40.2)	5.7
Dividend withholding tax and other cash taxes		1.3
Foreign exchange	0.6
Impairments		7.7
Income tax (benefit) expense	(24.2)	9.8
US
Reconciliation of income taxes
Computed income tax expense (benefit) at Canadian statutory rate	11.2	(2.2)
Operating in countries with different income tax rates	(0.3)	0.1
Income tax benefit after adjustments for countries with different income tax rates	10.9	(2.1)
Change in valuation allowance	(39.7)	(2.2)
Income tax benefit after adjustments for countries with different income tax rates and valuation allowance	(28.8)	(4.3)
Dividend withholding tax and other cash taxes	1.8	1.1
Changes in tax rates	(0.1)	2.2
Changes in estimates due to tax filings	3.0	(0.1)
Capital gain on intercompany notes		0.1
Impairments		7.7
Other	(1.2)	2.9
Total other reconciliation items	3.5	13.9
Income tax (benefit) expense	$ (25.3)	$ 9.6
Canada.
Reconciliation of income taxes
Effective income tax rate	27.00%	27.00%
Computed income tax expense (benefit) at Canadian statutory rate	$ 2.1	$ (7.0)
Income tax benefit after adjustments for countries with different income tax rates	2.1	(7.0)
Change in valuation allowance	(0.5)	7.9
Income tax benefit after adjustments for countries with different income tax rates and valuation allowance	1.6	0.9
Dividend withholding tax and other cash taxes	0.2	0.2
Foreign exchange	(0.6)	1.7
Changes in estimates due to tax filings		(0.2)
Capital gain on intercompany notes	0.2
Other	(0.3)	(2.4)
Total other reconciliation items	(0.5)	(0.7)
Income tax (benefit) expense	$ 1.1	$ 0.2