Accounting for derivative instruments and hedging activities - gain, loss (Details) - Derivative instruments not designated as cash flow hedges - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Change in fair value of derivatives
Impact of derivative instruments on the consolidated statements of operations
Unrealized gain (loss) resulting from changes in the fair value of derivative financial instruments	$ 6.8	$ (8.9)
Natural gas swaps
Impact of derivative instruments on the consolidated statements of operations
Realized loss (gain) for derivative instruments	2.5	0.9
Natural gas swaps | Change in fair value of derivatives
Impact of derivative instruments on the consolidated statements of operations
Unrealized gain (loss) resulting from changes in the fair value of derivative financial instruments	3.4	(4.6)
Gas purchase agreements
Impact of derivative instruments on the consolidated statements of operations
Realized loss (gain) for derivative instruments	8.3	8.2
Gas purchase agreements | Change in fair value of derivatives
Impact of derivative instruments on the consolidated statements of operations
Unrealized gain (loss) resulting from changes in the fair value of derivative financial instruments	5.4	3.2
Interest rate swaps
Impact of derivative instruments on the consolidated statements of operations
Realized loss (gain) for derivative instruments	5.0	(3.2)
Interest rate swaps | Change in fair value of derivatives
Impact of derivative instruments on the consolidated statements of operations
Unrealized gain (loss) resulting from changes in the fair value of derivative financial instruments	(2.0)	(7.5)
Convertible Debentures | Other (income) expense, net
Impact of derivative instruments on the consolidated statements of operations
Unrealized gain (loss) resulting from changes in the fair value of derivative financial instruments	$ (1.8)	$ 1.8