Accounting Changes Accounting Changes (Details) - CAD ($) $ in Millions	Mar. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Properties	$ 18,312	$ 18,406	$ 18,418
Other assets	471	470	71
Accounts payable and accrued liabilities	1,312	1,507	1,449
Other long-term liabilities	598	574	237
Deferred income taxes	3,549	3,515	3,518
Retained earnings	$ 6,798	6,630	$ 6,635
Accounting Standards Update 2016-02 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Properties		(12)
Other assets		399
Accounts payable and accrued liabilities		58
Other long-term liabilities		337
Deferred income taxes		(3)
Retained earnings		$ (5)