Income Taxes - Expected Income Tax Expense at Canadian Statutory Rates Reconciled to Income Tax Expense (Details) - CAD CAD in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Statutory federal and provincial income tax rate (Canada)	26.56%	26.65%	26.47%
Expected income tax expense at Canadian enacted statutory tax rates	CAD 663	CAD 573	CAD 519
Increase (decrease) in taxes resulting from:
(Gains) losses not subject to tax	(27)	(23)	28
Canadian tax rate differentials	1	0	1
Foreign tax rate differentials	(9)	0	39
Effect of tax rate (decrease) increase	(541)	0	23
Other	6	3	(3)
Income tax expense	CAD 93	CAD 553	CAD 607