Segmented and Geographic Information - Geographic Information (Details) - CAD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	$ 2,006	$ 1,898	$ 1,750	$ 1,662	$ 1,713	$ 1,595	$ 1,643	$ 1,603	$ 7,316	$ 6,554	$ 6,232
Long-term assets excluding financial instruments and pension assets	18,892				17,452				18,892	17,452	17,121
Canada
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues									5,232	4,667	4,473
Long-term assets excluding financial instruments and pension assets	12,133				11,505				12,133	11,505	11,000
United States [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues									2,084	1,887	1,759
Long-term assets excluding financial instruments and pension assets	$ 6,759				$ 5,947				$ 6,759	$ 5,947	$ 6,121