Other Comprehensive Income (Loss) and Accumulated Other Comprehensive Loss - Changes in Accumulated Other Comprehensive Loss by Component (Detail) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOC Income (Loss), Opening balance	$ (1,741)	$ (1,799)
Other comprehensive income (loss) before reclassifications	(394)	(71)
Amounts reclassified from accumulated other comprehensive loss	92	129
Net current-period other comprehensive income (loss)	(302)	58	$ (322)
AOC Income (Loss), Ending balance	(2,043)	(1,741)	(1,799)
Foreign currency net of hedging activities
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOC Income (Loss), Opening balance	109	127
Other comprehensive income (loss) before reclassifications	4	(17)
Amounts reclassified from accumulated other comprehensive loss	0	(1)
Net current-period other comprehensive income (loss)	4	(18)
AOC Income (Loss), Ending balance	113	109	127
Derivatives and other
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOC Income (Loss), Opening balance	(89)	(104)
Other comprehensive income (loss) before reclassifications	19	(4)
Amounts reclassified from accumulated other comprehensive loss	8	19
Net current-period other comprehensive income (loss)	27	15
AOC Income (Loss), Ending balance	(62)	(89)	(104)
Pension and post-retirement defined benefit plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOC Income (Loss), Opening balance	(1,761)	(1,822)
Other comprehensive income (loss) before reclassifications	(417)	(50)
Amounts reclassified from accumulated other comprehensive loss	84	111
Net current-period other comprehensive income (loss)	(333)	61
AOC Income (Loss), Ending balance	$ (2,094)	$ (1,761)	$ (1,822)