Segmented and Geographic Information - Geographic Information (Details) - CAD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues	$ 2,069	$ 1,979	$ 1,977	$ 1,767	$ 2,006	$ 1,898	$ 1,750	$ 1,662	$ 7,792	$ 7,316	$ 6,554
Long-term assets excluding financial instruments and pension assets	20,151				18,892				20,151	18,892	17,452
Canada
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues									5,675	5,232	4,667
Long-term assets excluding financial instruments and pension assets	13,131				12,133				13,131	12,133	11,505
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues									2,117	2,084	1,887
Long-term assets excluding financial instruments and pension assets	$ 7,020				$ 6,759				$ 7,020	$ 6,759	$ 5,947