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, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Statutory federal and provincial income tax rate (Canada)	26.65%	26.47%	26.31%
Expected income tax expense at Canadian enacted statutory tax rates	CAD 573	CAD 519	CAD 536
Increase (decrease) in taxes resulting from:
(Gains) losses not subject to tax	(23)	28	(5)
Canadian tax rate differentials	0	1	(1)
Foreign tax rate differentials	0	39	36
Effect of tax rate increases	0	23	0
Other	3	(3)	(4)
Income tax expense	CAD 553	CAD 607	CAD 562