Income Taxes - Summary of Major Components of Company's Income Tax Expense (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Current income tax expense	$ 381	$ 303	$ 233
Origination and reversal of temporary differences	214	371	336
Effect of tax rate decrease	(21)	(541)	0
Effect of hedge of net investment in foreign subsidiaries	64	(42)	(20)
Other	(1)	2	4
Total deferred income tax expense (recovery)	256	(210)	320
Income tax expense	637	93	553
Income before income tax expense
Canada	1,788	1,829	1,513
Foreign	800	669	639
Income before income tax expense	2,588	2,498	2,152
Current
Canada	336	257	165
Foreign	45	46	68
Total current income tax expense	381	303	233
Deferred
Canada	174	256	207
Foreign	82	(466)	113
Total deferred income tax expense (recovery)	256	(210)	320
Income tax expense	$ 637	$ 93	$ 553