Other Comprehensive Income (Loss) and Accumulated Other Comprehensive Loss - Amounts in Pension and Post-Retirement Defined Benefit Plans Reclassification from AOCL (Detail) - CAD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before income tax									$ 2,588	$ 2,498	$ 2,152
Income tax recovery									(637)	(93)	(553)
Total net of income tax	$ 545	$ 622	$ 436	$ 348	$ 984	$ 510	$ 480	$ 431	1,951	2,405	$ 1,599
Reclassification out of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amortization of prior service costs									(2)	(4)
Recognition of net actuarial loss									117	154
Total before income tax									115	150
Income tax recovery									(31)	(39)
Total net of income tax									$ 84	$ 111