Income Taxes - Summary of Major Components of Company's Income Tax Expense (Details) - CAD CAD in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Current income tax expense	CAD 303	CAD 233	CAD 373
Origination and reversal of temporary differences	371	336	105
Effect of tax rate (decrease) increase	(541)	0	23
Translation of the U.S. dollar-denominated long-term debt designated as a hedge of the net investment in U.S. subsidiaries	(42)	(20)	100
Other	2	4	6
Total deferred income tax (recovery) expense	(210)	320	234
Income tax expense	93	553	607
Income before income tax expense
Canada	1,829	1,513	1,099
Foreign	669	639	860
Income before income tax expense	2,498	2,152	1,959
Current
Canada	257	165	173
Foreign	46	68	200
Total current income tax expense	303	233	373
Deferred
Canada	256	207	163
Foreign	(466)	113	71
Total deferred income tax (recovery) expense	(210)	320	234
Income tax expense	CAD 93	CAD 553	CAD 607