PENSION AND OTHER POSTRETIREMENT BENEFITS - Net Benefit Costs Recognized (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Pension | Canada
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost1	$ 103	$ 81	$ 131
Interest cost	186	184	127
Expected return on plan assets	(304)	(271)	(295)
Amortization/settlement of net actuarial (gain)/loss	4	0	8
Amortization/curtailment of prior service credit	0	0	0
Net periodic benefit (credit)/cost	(11)	(6)	(29)
Defined contribution benefit cost	12	12	10
Net pension (credit)/cost recognized in Earnings	1	6	(19)
Amount recognized in OCI:
Amortization/settlement of net actuarial (gain)/loss	0	0	(2)
Amortization/curtailment of prior service credit	0	0	0
Net actuarial (gain)/loss arising during the year	(173)	115	(288)
Total amount recognized in OCI	(173)	115	(290)
Total amount recognized in Comprehensive income	(172)	121	(309)
Pension | US
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost1	60	40	43
Interest cost	69	47	24
Expected return on plan assets	(131)	(77)	(85)
Amortization/settlement of net actuarial (gain)/loss	7	(4)	0
Amortization/curtailment of prior service credit	(4)	0	(2)
Net periodic benefit (credit)/cost	1	6	(20)
Defined contribution benefit cost	0	0	0
Net pension (credit)/cost recognized in Earnings	1	6	(20)
Amount recognized in OCI:
Amortization/settlement of net actuarial (gain)/loss	0	4	0
Amortization/curtailment of prior service credit	4	0	2
Net actuarial (gain)/loss arising during the year	(116)	30	(52)
Total amount recognized in OCI	(112)	34	(50)
Total amount recognized in Comprehensive income	(111)	40	(70)
OPEB | Canada
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost1	3	3	4
Interest cost	10	11	7
Expected return on plan assets	0	0	0
Amortization/settlement of net actuarial (gain)/loss	(5)	(6)	(1)
Amortization/curtailment of prior service credit	(1)	0	0
Net pension (credit)/cost recognized in Earnings	7	8	10
Amount recognized in OCI:
Amortization/settlement of net actuarial (gain)/loss	5	6	1
Amortization/curtailment of prior service credit	1	0	0
Net actuarial (gain)/loss arising during the year	(22)	13	(67)
Total amount recognized in OCI	(16)	19	(66)
Total amount recognized in Comprehensive income	(9)	27	(56)
OPEB | US
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost1	3	1	1
Interest cost	7	6	3
Expected return on plan assets	(15)	(11)	(12)
Amortization/settlement of net actuarial (gain)/loss	(5)	(6)	(6)
Amortization/curtailment of prior service credit	(6)	(8)	(7)
Net pension (credit)/cost recognized in Earnings	(16)	(18)	(21)
Amount recognized in OCI:
Amortization/settlement of net actuarial (gain)/loss	5	6	6
Amortization/curtailment of prior service credit	6	8	7
Net actuarial (gain)/loss arising during the year	(12)	0	(4)
Total amount recognized in OCI	(1)	14	9
Total amount recognized in Comprehensive income	$ (17)	$ (4)	$ (12)