Accounting for derivative instruments - forecasted trans (Details) kJ in Millions, MMBTU in Millions, CAD in Millions, $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017 CAD MMBTU kJ	Dec. 31, 2016 USD ($) MMBTU kJ	Sep. 30, 2017 USD ($)
Natural gas swaps
Derivative instruments
Volume of forecasted transactions, energy (swaps: Mmbtu, agreements: joules) | MMBTU	9.0	4.9
Gas purchase agreements
Derivative instruments
Volume of forecasted transactions, energy (swaps: Mmbtu, agreements: joules) | kJ	10.5	11.3
Interest rate swaps
Derivative instruments
Volume of forecasted transactions, (in dollars) | $		$ 506.9	$ 434.6
Foreign currency forward contracts
Derivative instruments
Volume of forecasted transactions, (in dollars) | CAD	CAD 35.0