Accounting for derivative instruments - forecasted trans (Details) kJ in Millions, MMBTU in Millions, $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2016 USD ($) kJ MMBTU	Dec. 31, 2015 USD ($) kJ MMBTU
Natural gas swaps
Derivative instruments
Volume of forecasted transactions, energy (swaps: Mmbtu, agreements: joules) | MMBTU	5.7	2.8
Gas purchase agreements
Derivative instruments
Volume of forecasted transactions, energy (swaps: Mmbtu, agreements: joules) | kJ	19.4	25.0
Interest rate swaps
Derivative instruments
Volume of forecasted transactions, (in dollars) | $	$ 532.3	$ 302.3