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, 2020	Dec. 31, 2019
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	$ (2,522)	$ (2,043)
Other comprehensive loss before reclassifications	(432)	(551)
Amounts reclassified from accumulated other comprehensive loss	140	72
Net other comprehensive income (loss)	(292)	(479)
AOC Income (Loss), Ending balance	(2,814)	(2,522)
Foreign currency net of hedging activities
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	112	113
Other comprehensive loss before reclassifications	0	(1)
Amounts reclassified from accumulated other comprehensive loss	0	0
Net other comprehensive income (loss)	0	(1)
AOC Income (Loss), Ending balance	112	112
Derivatives and other
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	(54)	(62)
Other comprehensive loss before reclassifications	(2)	0
Amounts reclassified from accumulated other comprehensive loss	8	8
Net other comprehensive income (loss)	6	8
AOC Income (Loss), Ending balance	(48)	(54)
Pension and post-retirement defined benefit plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	(2,580)	(2,094)
Other comprehensive loss before reclassifications	(430)	(550)
Amounts reclassified from accumulated other comprehensive loss	132	64
Net other comprehensive income (loss)	(298)	(486)
AOC Income (Loss), Ending balance	$ (2,878)	$ (2,580)