PENSION AND OTHER POSTRETIREMENT BENEFITS - NET BENEFIT COSTS RECOGNIZED (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Pension | Canada
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	$ 139	$ 148	$ 149
Interest cost	101	128	139
Expected return on plan assets	(252)	(260)	(245)
Amortization/settlement of net actuarial loss	54	42	41
Amortization/curtailment of prior service credit	0	0	0
Net periodic benefit (credit)/cost	42	58	84
Defined contribution benefit cost	7	6	8
Net pension (credit)/cost recognized in Earnings	49	64	92
Amount recognized in OCI:
Effect of plan combination	0	0	0
Amortization/settlement of net actuarial loss	(25)	(21)	(26)
Amortization/curtailment of prior service credit	0	0	0
Net actuarial (gain)/loss arising during the year	(291)	118	115
Total amount recognized in OCI	(316)	97	89
Total amount recognized in Comprehensive income	(267)	161	181
Pension | United States
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	44	44	45
Interest cost	17	31	41
Expected return on plan assets	(73)	(88)	(78)
Amortization/settlement of net actuarial loss	11	1	2
Amortization/curtailment of prior service credit	0	(1)	(1)
Net periodic benefit (credit)/cost	(1)	(13)	9
Defined contribution benefit cost	0	0	0
Net pension (credit)/cost recognized in Earnings	(1)	(13)	9
Amount recognized in OCI:
Effect of plan combination	0	0	(6)
Amortization/settlement of net actuarial loss	(11)	(1)	(2)
Amortization/curtailment of prior service credit	0	1	1
Net actuarial (gain)/loss arising during the year	(99)	100	8
Total amount recognized in OCI	(110)	100	1
Total amount recognized in Comprehensive income	(111)	87	10
OPEB | Canada
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	6	5	5
Interest cost	7	8	10
Expected return on plan assets	0	0	0
Amortization/settlement of net actuarial loss	0	(1)	(7)
Amortization/curtailment of prior service credit	0	0	(1)
Net pension (credit)/cost recognized in Earnings	13	12	7
Amount recognized in OCI:
Amortization/settlement of net actuarial loss	0	1	7
Amortization/curtailment of prior service credit	0	0	1
Net actuarial (gain)/loss arising during the year	(50)	21	15
Prior service credit	0	0	0
Total amount recognized in OCI	(50)	22	23
Total amount recognized in Comprehensive income	(37)	34	30
OPEB | United States
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	1	2	2
Interest cost	3	7	10
Expected return on plan assets	(10)	(12)	(12)
Amortization/settlement of net actuarial loss	(1)	(1)	0
Amortization/curtailment of prior service credit	(7)	(2)	(2)
Net pension (credit)/cost recognized in Earnings	(14)	(6)	(2)
Amount recognized in OCI:
Amortization/settlement of net actuarial loss	1	1	0
Amortization/curtailment of prior service credit	7	2	2
Net actuarial (gain)/loss arising during the year	(80)	15	(8)
Prior service credit	0	(33)	0
Total amount recognized in OCI	(72)	(15)	(6)
Total amount recognized in Comprehensive income	$ (86)	$ (21)	$ (8)