Income Tax Expense (Credit) for Components of Other Comprehensive Income (Loss) (Detail) - CAD CAD in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Post-retirement benefits liability adjustment (excluding amortization)			CAD 16	CAD 37
Amortization of post-retirement benefits liability adjustment included in net periodic benefit cost	CAD 13	CAD 15	26	20
Total	CAD 13	CAD 15	CAD 42	CAD 57