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Derivatives and Risk Management Activities - Currency Exchange Rate Risk Hedging (Details)
$ in Millions, $ in Millions
Sep. 30, 2018
USD ($)
$ / $
Sep. 30, 2018
CAD ($)
$ / $
Forward exchange contracts that exchange CAD for USD maturing in 2018    
Currency Exchange Rate Risk Hedging:    
Notional amount of derivatives $ 174 $ 227
Average exchange rate (cad per usd) 1.30 1.30
Forward exchange contracts that exchange USD for CAD maturing in 2018    
Currency Exchange Rate Risk Hedging:    
Notional amount of derivatives $ 418 $ 543
Average exchange rate (cad per usd) 1.30 1.30
Forward exchange contracts that exchange USD for CAD maturing in 2019    
Currency Exchange Rate Risk Hedging:    
Notional amount of derivatives $ 120 $ 155
Average exchange rate (cad per usd) 1.30 1.30