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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before income tax									CAD 2,498	CAD 2,152	CAD 1,959
Income tax recovery									(93)	(553)	(607)
Net of income tax	CAD 984	CAD 510	CAD 480	CAD 431	CAD 384	CAD 347	CAD 328	CAD 540	2,405	1,599	CAD 1,352
Reclassification out of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amortization of prior service costs									(4)	(6)
Recognition of net actuarial loss									154	194
Total before income tax									150	188
Income tax recovery									(39)	(50)
Net of income tax									CAD 111	CAD 138