Income tax expense - Deferred tax assets and liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
Loss carryforwards	$ 123.9	$ 135.9
Capital loss carryforwards	35.3	35.8
Interest expense limitation carryforwards		9.7
Finance and share issuance costs		0.1
Tax Credits	1.4	1.4
Stock-based compensation	2.4	2.4
Derivative contracts	3.8	5.7
Other long-term notes	2.3
Other	3.1	0.9
Total deferred tax assets	172.2	191.9
Less: Valuation allowance	(105.2)	(145.4)
Net deferred tax assets	67.0	46.5
Deferred tax liabilities:
Intangible assets	(22.0)	(21.9)
Property, plant and equipment	(22.0)	(31.2)
Basis difference in joint ventures	(5.8)	(5.4)
Other long-term investments		(1.3)
Total deferred tax liabilities	(49.8)	(59.8)
Net deferred tax liability		(13.3)
Net deferred tax asset (liability)	17.2
US
Deferred tax liabilities:
Net deferred tax liability		(23.7)
Net deferred tax asset (liability)	4.5
Canada.
Deferred tax liabilities:
Net deferred tax asset (liability)	$ 12.7	$ 10.4