Other Comprehensive Income (Loss) and Accumulated Other Comprehensive Loss - Amounts in Pension and Post-Retirement Defined Benefit Plans Reclassification from AOCL (Detail) - CAD CAD in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income tax expense									CAD 2,152	CAD 1,959	CAD 2,038
Income tax recovery									(553)	(607)	(562)
Net income	CAD 384	CAD 347	CAD 328	CAD 540	CAD 319	CAD 323	CAD 390	CAD 320	1,599	1,352	CAD 1,476
Reclassification out of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amortization of prior service costs(1)									(6)	(5)
Recognition of net actuarial loss(1)									194	269
Income before income tax expense									188	264
Income tax recovery									(50)	(71)
Net income									CAD 138	CAD 193