PENSION AND OTHER POSTRETIREMENT BENEFITS - Net Benefit Costs Recognized (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Pension | Canada
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	$ 81	$ 131	$ 139
Interest cost	184	127	101
Expected return on plan assets	(271)	(295)	(252)
Amortization/settlement of net actuarial loss	0	8	54
Amortization/curtailment of prior service credit	0	0	0
Net periodic benefit (credit)/cost	(6)	(29)	42
Defined contribution benefit cost	12	10	7
Net pension (credit)/cost recognized in Earnings	6	(19)	49
Amount recognized in OCI:
Amortization/settlement of net actuarial (gain)/loss	0	(2)	(25)
Amortization/curtailment of prior service credit	0	0	0
Net actuarial (gain)/loss arising during the year	115	(288)	(291)
Total amount recognized in OCI	115	(290)	(316)
Total amount recognized in Comprehensive income	121	(309)	(267)
Pension | US
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	40	43	44
Interest cost	47	24	17
Expected return on plan assets	(77)	(85)	(73)
Amortization/settlement of net actuarial loss	(4)	0	11
Amortization/curtailment of prior service credit	0	(2)	0
Net periodic benefit (credit)/cost	6	(20)	(1)
Defined contribution benefit cost	0	0	0
Net pension (credit)/cost recognized in Earnings	6	(20)	(1)
Amount recognized in OCI:
Amortization/settlement of net actuarial (gain)/loss	4	0	(11)
Amortization/curtailment of prior service credit	0	2	0
Net actuarial (gain)/loss arising during the year	30	(52)	(99)
Total amount recognized in OCI	34	(50)	(110)
Total amount recognized in Comprehensive income	40	(70)	(111)
OPEB | Canada
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	3	4	6
Interest cost	11	7	7
Expected return on plan assets	0	0	0
Amortization/settlement of net actuarial loss	(6)	(1)	0
Amortization/curtailment of prior service credit	0	0	0
Net pension (credit)/cost recognized in Earnings	8	10	13
Amount recognized in OCI:
Amortization/settlement of net actuarial (gain)/loss	6	1	0
Amortization/curtailment of prior service credit	0	0	0
Net actuarial (gain)/loss arising during the year	13	(67)	(50)
Total amount recognized in OCI	19	(66)	(50)
Total amount recognized in Comprehensive income	27	(56)	(37)
OPEB | US
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	1	1	1
Interest cost	6	3	3
Expected return on plan assets	(11)	(12)	(10)
Amortization/settlement of net actuarial loss	(6)	(6)	(1)
Amortization/curtailment of prior service credit	(8)	(7)	(7)
Net pension (credit)/cost recognized in Earnings	(18)	(21)	(14)
Amount recognized in OCI:
Amortization/settlement of net actuarial (gain)/loss	6	6	1
Amortization/curtailment of prior service credit	8	7	7
Net actuarial (gain)/loss arising during the year	0	(4)	(80)
Total amount recognized in OCI	14	9	(72)
Total amount recognized in Comprehensive income	$ (4)	$ (12)	$ (86)