Other Comprehensive Income (Loss) and Accumulated Other Comprehensive Loss - Changes in Accumulated Other Comprehensive Loss by Component (Detail) - CAD CAD in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOC Income (Loss), Opening balance	CAD (1,799)	CAD (1,477)
Other comprehensive (loss) income before reclassifications	(71)	(468)
Amounts reclassified from accumulated other comprehensive loss	129	146
Net current-period other comprehensive income (loss)	58	(322)	CAD 742
AOC Income (Loss), Ending balance	(1,741)	(1,799)	(1,477)
Foreign currency net of hedging activities
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOC Income (Loss), Opening balance	127	129
Other comprehensive (loss) income before reclassifications	(17)	(2)
Amounts reclassified from accumulated other comprehensive loss	(1)	0
Net current-period other comprehensive income (loss)	(18)	(2)
AOC Income (Loss), Ending balance	109	127	129
Derivatives and other
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOC Income (Loss), Opening balance	(104)	(102)
Other comprehensive (loss) income before reclassifications	(4)	(10)
Amounts reclassified from accumulated other comprehensive loss	19	8
Net current-period other comprehensive income (loss)	15	(2)
AOC Income (Loss), Ending balance	(89)	(104)	(102)
Pension and post-retirement defined benefit plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOC Income (Loss), Opening balance	(1,822)	(1,504)
Other comprehensive (loss) income before reclassifications	(50)	(456)
Amounts reclassified from accumulated other comprehensive loss	111	138
Net current-period other comprehensive income (loss)	61	(318)
AOC Income (Loss), Ending balance	CAD (1,761)	CAD (1,822)	CAD (1,504)