Derivatives and Risk Management Activities - Currency Exchange Rate Risk Hedging (Details) - Dec. 31, 2016 CAD in Millions, $ in Millions	USD ($) contract CAD / $	CAD contract CAD / $
Forward exchange contracts that exchange CAD for USD at the rate USD 1.00 to CAD 1.38 maturing in 2016
Currency Exchange Rate Risk Hedging:
Notional Amount	$ 274	CAD 363
Average exchange rate | CAD / $	1.33	1.33
Forward exchange contracts that exchange USD for CAD at the rate USD 1.00 to CAD 1.33 maturing in 2016
Currency Exchange Rate Risk Hedging:
Notional Amount	$ 492	CAD 652
Average exchange rate | CAD / $	1.33	1.33
Cash Flow Hedging | 8 forward starting 30-year interest rate swap 1
Currency Exchange Rate Risk Hedging:
Number of interest rate derivatives held | contract	8	8
Notional Amount | $	$ 200
Average Rate Locked	3.14%	3.14%
Cash Flow Hedging | 8 forward starting 30-year interest rate swap 2
Currency Exchange Rate Risk Hedging:
Number of interest rate derivatives held | contract	8	8
Notional Amount | $	$ 200
Average Rate Locked	3.20%	3.20%
Cash Flow Hedging | 8 forward starting 30-year interest rate swap 3
Currency Exchange Rate Risk Hedging:
Number of interest rate derivatives held | contract	8	8
Notional Amount | $	$ 200
Average Rate Locked	2.83%	2.83%