Accounting for derivative instruments and hedging activities - forecasted transactions (Details) kJ in Millions, MMBTU in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) MMBTU kJ	Dec. 31, 2019 USD ($) kJ MMBTU
Natural gas swaps
Derivative instruments
Volume of forecasted transactions, energy (swaps: Mmbtu, agreements: joules) | MMBTU	12.4	16.3
Gas purchase agreements
Derivative instruments
Volume of forecasted transactions, energy (swaps: Mmbtu, agreements: joules) | kJ	4.0	6.4
Interest rate swaps
Derivative instruments
Volume of forecasted transactions, (in dollars) | $	$ 122.3	$ 468.4