Accounting Changes - Impact of ASC 326 Adoption (Details) - CAD ($) $ in Millions	Sep. 30, 2020	Jun. 30, 2020	Jan. 01, 2020	Dec. 31, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Impact of the adoption of ASC 326	$ (40)	$ (40)		$ (43)
Cumulative Effect, Period of Adoption, Adjustment | Accounting Standards Update 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Impact of the adoption of ASC 326			$ 1