Segmented and Geographic Information - Geographic Information (Details) - CAD CAD in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	CAD 1,713	CAD 1,595	CAD 1,643	CAD 1,603	CAD 1,637	CAD 1,554	CAD 1,450	CAD 1,591	CAD 6,554	CAD 6,232	CAD 6,712
Long-term assets excluding financial instruments and pension assets	17,452				17,121				17,452	17,121	16,698
Canada [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues									4,667	4,473	4,662
Long-term assets excluding financial instruments and pension assets	11,505				11,000				11,505	11,000	10,630
United States [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues									1,887	1,759	2,050
Long-term assets excluding financial instruments and pension assets	CAD 5,947				CAD 6,121				CAD 5,947	CAD 6,121	CAD 6,068