Change in Projected Benefit Obligation (Detail) - CAD CAD in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension benefits
Schedule Of Defined Benefit Plans Disclosures [Line Items]
Projected benefit obligation at January 1		CAD 8,356	CAD 8,147
Current service cost		217	203	CAD 211
Interest cost		313	319	307
Actuarial loss (gain)		415	157
Benefits paid	[1]	(516)	(470)
Projected benefit obligation at December 31		8,785	8,356	8,147
Accumulated benefit obligation at December 31		8,043	7,681
Other post-retirement benefits
Schedule Of Defined Benefit Plans Disclosures [Line Items]
Projected benefit obligation at January 1		706	642
Current service cost		16	16	15
Interest cost		23	27	25
Actuarial loss (gain)		(49)	46
Benefits paid	[1]	(26)	(25)
Projected benefit obligation at December 31		CAD 670	CAD 706	CAD 642

[1] Benefit payments for funded and unfunded plans.