Changes in Accumulated Other Comprehensive Loss ("AOCL") by Component - Changes in Accumulated Other Comprehensive Loss (AOCL) by Component (Details) - CAD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	$ (2,814)	$ (2,522)
Other comprehensive income before reclassifications	17	7
Amounts reclassified from accumulated other comprehensive loss	41	35
Net other comprehensive income	58	42
AOC Income (Loss), Closing balance	(2,756)	(2,480)
Foreign currency net of hedging activities
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	112	112
Other comprehensive income before reclassifications	0	7
Amounts reclassified from accumulated other comprehensive loss	0	0
Net other comprehensive income	0	7
AOC Income (Loss), Closing balance	112	119
Derivatives and other
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	(48)	(54)
Other comprehensive income before reclassifications	17	0
Amounts reclassified from accumulated other comprehensive loss	2	2
Net other comprehensive income	19	2
AOC Income (Loss), Closing balance	(29)	(52)
Pension and post-retirement defined benefit plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	(2,878)	(2,580)
Other comprehensive income before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive loss	39	33
Net other comprehensive income	39	33
AOC Income (Loss), Closing balance	$ (2,839)	$ (2,547)