Accounting for derivative instruments and hedging activities - FV by Bal Sht Loc (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Fair value of derivative instruments
Derivative Assets		$ 0.7
Derivative Liabilities	$ 31.4	27.9
Derivative instruments designated as cash flow hedges
Fair value of derivative instruments
Derivative Liabilities	1.9	1.5
Derivative instruments designated as cash flow hedges | Interest rate swaps | Current
Fair value of derivative instruments
Derivative Liabilities	0.6	0.4
Derivative instruments designated as cash flow hedges | Interest rate swaps | Long-term
Fair value of derivative instruments
Derivative Liabilities	1.3	1.1
Derivative instruments not designated as cash flow hedges
Fair value of derivative instruments
Derivative Assets		0.7
Derivative Liabilities	29.5	26.4
Derivative instruments not designated as cash flow hedges | Interest rate swaps | Current
Fair value of derivative instruments
Derivative Liabilities	5.4	1.9
Derivative instruments not designated as cash flow hedges | Interest rate swaps | Long-term
Fair value of derivative instruments
Derivative Liabilities	3.1	1.1
Derivative instruments not designated as cash flow hedges | Natural gas swaps | Current
Fair value of derivative instruments
Derivative Liabilities	2.0	1.9
Derivative instruments not designated as cash flow hedges | Natural gas swaps | Long-term
Fair value of derivative instruments
Derivative Liabilities	3.5	4.2
Derivative instruments not designated as cash flow hedges | Gas purchase agreements | Current
Fair value of derivative instruments
Derivative Assets		0.7
Derivative Liabilities	4.1	4.6
Derivative instruments not designated as cash flow hedges | Gas purchase agreements | Long-term
Fair value of derivative instruments
Derivative Liabilities	6.2	9.5
Derivative instruments not designated as cash flow hedges | Convertible Debentures | Current
Fair value of derivative instruments
Derivative Liabilities	$ 5.2	$ 3.2