Derivatives and Risk Management Activities - Currency Exchange Rate Risk Hedging (Details) - 12 months ended Dec. 31, 2018 $ in Millions, $ in Millions	USD ($) contract $ / $	CAD ($) contract $ / $
2.83%, 8 forward starting swaps (30-year) | Cash Flow Hedging
Currency Exchange Rate Risk Hedging:
Number of interest rate derivatives | contract	8	8
Term of derivative contract	30 years
Notional amount of derivatives | $	$ 200
Average rate locked (percent)	2.83%	2.83%
3.06%, 8 forward starting swaps (30-year) | Cash Flow Hedging
Currency Exchange Rate Risk Hedging:
Number of interest rate derivatives | contract	8	8
Term of derivative contract	30 years
Notional amount of derivatives | $	$ 200
Average rate locked (percent)	3.06%	3.06%
Forward exchange contracts that exchange CAD for USD maturing in 2019
Currency Exchange Rate Risk Hedging:
Notional amount of derivatives	$ 3	$ 4
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	$ 218	$ 284
Average exchange rate (cad per usd) | $ / $	1.31	1.31