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Derivatives and Risk Management Activities - Currency Exchange Rate Risk Hedging (Details)
$ in Millions, $ in Millions
Mar. 31, 2019
USD ($)
$ / $
Mar. 31, 2019
CAD ($)
$ / $
Forward exchange contracts that exchange CAD for USD maturing in 2019    
Currency Exchange Rate Risk Hedging:    
Notional amount of derivatives $ 4 $ 5
Average exchange rate (cad per usd) 1.34 1.34
Forward exchange contracts that exchange USD for CAD maturing in 2019    
Currency Exchange Rate Risk Hedging:    
Notional amount of derivatives $ 144 $ 191
Average exchange rate (cad per usd) 1.33 1.33