Income taxes (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Components of income tax expenses (benefit)
Current income tax expense	$ 0.3	$ 6.2	$ 1.8	$ 7.3
Deferred tax benefit	(18.7)	(3.3)	(18.6)	(9.0)
Total income tax (benefit) expense, net	$ (18.4)	$ 2.9	$ (16.8)	$ (1.7)
Reconciliation of income taxes
Canadian enacted statutory rate (as a percent)	26.00%	26.00%	26.00%	26.00%
Computed income taxes at Canadian statutory rate	$ 9.0	$ 4.4	$ 12.0	$ 0.8
Capital gain on intercompany notes	4.6		4.2
Foreign exchange	2.6	2.4	5.1	4.1
Change in valuation allowance	1.8	9.0	4.6	6.2
Dividend withholding tax and other cash taxes		3.4		3.6
Other permanent differences	0.4	2.5	0.1	0.6
Tax credit		3.6		3.6
Capital losses on tax restructuring	18.8		18.8
Countries with different tax rates		$ 1.6		$ 4.0
Valuation allowance	$ 179.8		$ 179.8