Accounting Changes - Impact of ASC 842 Adoption (Details) - CAD ($) $ in Millions	Jun. 30, 2019	Jan. 01, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Properties	$ 18,489	$ 18,406	$ 18,418
Other assets	466	470	71
Accounts payable and accrued liabilities	1,401	1,507	1,449
Other long-term liabilities	598	574	237
Deferred income taxes	3,525	3,515	3,518
Retained earnings	$ 7,125	6,630	$ 6,635
Accounting Standards Update 2016-02
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)