Segmented and Geographic Information - Geographic Information (Details) - CAD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues	$ 2,012	$ 1,863	$ 1,792	$ 2,043	$ 2,069	$ 1,979	$ 1,977	$ 1,767	$ 7,710	$ 7,792	$ 7,316
Long-term assets excluding financial instruments and pension assets	21,423				20,151				21,423	20,151	18,892
Canada
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues									5,829	5,675	5,232
Long-term assets excluding financial instruments and pension assets	14,258				13,131				14,258	13,131	12,133
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues									1,881	2,117	2,084
Long-term assets excluding financial instruments and pension assets	$ 7,165				$ 7,020				$ 7,165	$ 7,020	$ 6,759