Derivatives and Risk Management Activities - Currency Exchange Rate Risk Hedging (Details) CAD in Millions, $ in Millions	Mar. 31, 2017 CAD CAD / $	Mar. 31, 2017 USD ($) CAD / $
Forward exchange contracts that exchange CAD For USD maturing 2017
Currency Exchange Rate Risk Hedging:
Notional amount of derivatives	CAD 234	$ 175
Average exchange rate (cad per usd)	1.34	1.34
Forward exchange contracts that exchange USD for CAD maturing in 2017
Currency Exchange Rate Risk Hedging:
Notional amount of derivatives	CAD 569	$ 428
Average exchange rate (cad per usd)	1.33	1.33