Accounting Changes - Narrative (Details) - CAD ($) $ in Millions	Dec. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
Change in Accounting Estimate [Line Items]
Properties	$ 19,156	$ 18,406	$ 18,418
Other assets	451	470	71
Accounts payable and accrued liabilities	1,693	1,507	1,449
Other long-term liabilities	562	574	237
Deferred income taxes	3,501	3,515	3,518
Retained earnings	$ 7,550		6,635
Accounting Standards Update 2016-02
Change in Accounting Estimate [Line Items]
Properties		(12)
Other assets		399
Accounts payable and accrued liabilities		58
Other long-term liabilities		337
Deferred income taxes		(3)
Retained earnings
Change in Accounting Estimate [Line Items]
Retained earnings		6,630	$ 6,635
Retained earnings | Accounting Standards Update 2016-02
Change in Accounting Estimate [Line Items]
Retained earnings		$ (5)