Accounting for derivative instruments and hedging activities - forecasted transactions (Details) kJ in Millions, MMBTU in Millions, $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2020 USD ($) kJ MMBTU	Dec. 31, 2019 USD ($) MMBTU kJ
Natural gas swaps
Derivative instruments
Volume of forecasted transactions, energy (swaps: Mmbtu, agreements: joules) | MMBTU	14.5	16.3
Gas purchase agreements
Derivative instruments
Volume of forecasted transactions, energy (swaps: Mmbtu, agreements: joules) | kJ	6.6	6.4
Interest rate swaps
Derivative instruments
Volume of forecasted transactions, (in dollars) | $	$ 441.4	$ 468.4