PENSION AND OTHER POSTRETIREMENT BENEFITS - Net Benefit Costs Recognized (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Pension | Canada
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	$ 131	$ 139	$ 148
Interest cost	127	101	128
Expected return on plan assets	(295)	(252)	(260)
Amortization/settlement of net actuarial loss	8	54	42
Amortization/curtailment of prior service credit	0	0	0
Net periodic benefit (credit)/cost	(29)	42	58
Defined contribution benefit cost	10	7	6
Net pension (credit)/cost recognized in Earnings	(19)	49	64
Amount recognized in OCI:
Amortization/settlement of net actuarial loss	(2)	(25)	(21)
Amortization/curtailment of prior service credit	0	0	0
Net actuarial (gain)/loss arising during the year	(288)	(291)	118
Total amount recognized in OCI	(290)	(316)	97
Total amount recognized in Comprehensive income	(309)	(267)	161
Pension | United States
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	43	44	44
Interest cost	24	17	31
Expected return on plan assets	(85)	(73)	(88)
Amortization/settlement of net actuarial loss	0	11	1
Amortization/curtailment of prior service credit	(2)	0	(1)
Net periodic benefit (credit)/cost	(20)	(1)	(13)
Defined contribution benefit cost	0	0	0
Net pension (credit)/cost recognized in Earnings	(20)	(1)	(13)
Amount recognized in OCI:
Amortization/settlement of net actuarial loss	0	(11)	(1)
Amortization/curtailment of prior service credit	2	0	1
Net actuarial (gain)/loss arising during the year	(52)	(99)	100
Total amount recognized in OCI	(50)	(110)	100
Total amount recognized in Comprehensive income	(70)	(111)	87
OPEB | Canada
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	4	6	5
Interest cost	7	7	8
Expected return on plan assets	0	0	0
Amortization/settlement of net actuarial loss	(1)	0	(1)
Amortization/curtailment of prior service credit	0	0	0
Net pension (credit)/cost recognized in Earnings	10	13	12
Amount recognized in OCI:
Amortization/settlement of net actuarial loss	1	0	1
Amortization/curtailment of prior service credit	0	0	0
Net actuarial (gain)/loss arising during the year	(67)	(50)	21
Prior service credit	0	0	0
Total amount recognized in OCI	(66)	(50)	22
Total amount recognized in Comprehensive income	(56)	(37)	34
OPEB | United States
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	1	1	2
Interest cost	3	3	7
Expected return on plan assets	(12)	(10)	(12)
Amortization/settlement of net actuarial loss	(6)	(1)	(1)
Amortization/curtailment of prior service credit	(7)	(7)	(2)
Net pension (credit)/cost recognized in Earnings	(21)	(14)	(6)
Amount recognized in OCI:
Amortization/settlement of net actuarial loss	6	1	1
Amortization/curtailment of prior service credit	7	7	2
Net actuarial (gain)/loss arising during the year	(4)	(80)	15
Prior service credit	0	0	(33)
Total amount recognized in OCI	9	(72)	(15)
Total amount recognized in Comprehensive income	$ (12)	$ (86)	$ (21)