Segmented Information (Goodwill, Property, Plant And Equipment And Total Assets By Geographic Region) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting, Asset Reconciling Item [Line Items]
Goodwill, Balance End of Period	$ 2,609	$ 2,779
Property, Plant and Equipment	8,954	8,139
Total Assets	15,267	14,653
Canada [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Goodwill, Balance End of Period	696	650	$ 661
Property, Plant and Equipment	2,319	2,000
Total Assets	5,412	4,732
United States [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Goodwill, Balance End of Period	1,913	2,129	$ 2,129
Property, Plant and Equipment	6,635	6,139
Total Assets	9,811	9,902
Other Countries [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Total Assets	$ 44	$ 19