Segmented Information (Goodwill, Property, Plant And Equipment And Total Assets By Segment) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting, Asset Reconciling Item [Line Items]
Goodwill	$ 2,609	$ 2,779
Property, Plant and Equipment	8,954	8,139
Total Assets	15,267	14,653
Operating Segments [Member] | Canadian Operations [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Goodwill	696	650
Property, Plant and Equipment	862	602
Total Assets	1,908	1,542
Operating Segments [Member] | USA Operations [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Goodwill	1,913	2,129
Property, Plant and Equipment	6,555	6,050
Total Assets	9,301	9,535
Operating Segments [Member] | Market Optimization [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Property, Plant and Equipment	2	2
Total Assets	152	105
Corporate, Non-segment [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Property, Plant and Equipment	1,535	1,485
Total Assets	$ 3,906	$ 3,471