Other Comprehensive Income (Loss) and Accumulated Other Comprehensive Loss - Changes in Accumulated Other Comprehensive Loss by Component (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOC Income (Loss), Opening balance	$ (2,043)	$ (1,741)
Other comprehensive income (loss) before reclassifications	(551)	(394)
Amounts reclassified from accumulated other comprehensive loss	72	92
Net current-period other comprehensive income (loss)	(479)	(302)
AOC Income (Loss), Ending balance	(2,522)	(2,043)
Foreign currency net of hedging activities
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOC Income (Loss), Opening balance	113	109
Other comprehensive income (loss) before reclassifications	(1)	4
Amounts reclassified from accumulated other comprehensive loss	0	0
Net current-period other comprehensive income (loss)	(1)	4
AOC Income (Loss), Ending balance	112	113
Derivatives and other
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOC Income (Loss), Opening balance	(62)	(89)
Other comprehensive income (loss) before reclassifications	0	19
Amounts reclassified from accumulated other comprehensive loss	8	8
Net current-period other comprehensive income (loss)	8	27
AOC Income (Loss), Ending balance	(54)	(62)
Pension and post-retirement defined benefit plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOC Income (Loss), Opening balance	(2,094)	(1,761)
Other comprehensive income (loss) before reclassifications	(550)	(417)
Amounts reclassified from accumulated other comprehensive loss	64	84
Net current-period other comprehensive income (loss)	(486)	(333)
AOC Income (Loss), Ending balance	$ (2,580)	$ (2,094)