PENSION AND OTHER POSTRETIREMENT BENEFITS - NET BENEFIT COSTS RECOGNIZED (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension | Canada
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	$ 148	$ 149	$ 149
Interest cost	128	139	130
Expected return on plan assets	(260)	(245)	(245)
Amortization/settlement of net actuarial loss	42	41	25
Amortization/curtailment of prior service (credit)/cost	0	0	0
Net periodic benefit cost	58	84	59
Defined contribution benefit cost	6	8	11
Net pension cost recognized in Earnings	64	92	70
Amount recognized in OCI:
Effect of plan combination	0	0	0
Amortization/settlement of net actuarial loss	(21)	(26)	(11)
Amortization/curtailment of prior service credit/(cost)	0	0	0
Net actuarial loss arising during the year	118	115	112
Total amount recognized in OCI	97	89	101
Total amount recognized in Comprehensive income	161	181	171
Pension | United States
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	44	45	45
Interest cost	31	41	38
Expected return on plan assets	(88)	(78)	(88)
Amortization/settlement of net actuarial loss	1	2	7
Amortization/curtailment of prior service (credit)/cost	(1)	(1)	3
Net periodic benefit cost	(13)	9	5
Defined contribution benefit cost	0	0	0
Net pension cost recognized in Earnings	(13)	9	5
Amount recognized in OCI:
Effect of plan combination	0	(6)	0
Amortization/settlement of net actuarial loss	(1)	(2)	(7)
Amortization/curtailment of prior service credit/(cost)	1	1	(3)
Net actuarial loss arising during the year	100	8	28
Total amount recognized in OCI	100	1	18
Total amount recognized in Comprehensive income	87	10	23
OPEB | Canada
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	5	5	8
Interest cost	8	10	10
Expected return on plan assets	0	0	0
Amortization/settlement of net actuarial loss	(1)	(7)	0
Amortization/curtailment of prior service (credit)/cost	0	(1)	0
Net pension cost recognized in Earnings	12	7	18
Amount recognized in OCI:
Amortization/settlement of net actuarial loss	1	7	0
Amortization/curtailment of prior service credit/(cost)	0	1	0
Net actuarial loss arising during the year	21	15	(46)
Prior service credit	0	0	0
Total amount recognized in OCI	22	23	(46)
Total amount recognized in Comprehensive income	34	30	(28)
OPEB | United States
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	2	2	3
Interest cost	7	10	10
Expected return on plan assets	(12)	(12)	(12)
Amortization/settlement of net actuarial loss	(1)	0	(1)
Amortization/curtailment of prior service (credit)/cost	(2)	(2)	(4)
Net pension cost recognized in Earnings	(6)	(2)	(4)
Amount recognized in OCI:
Amortization/settlement of net actuarial loss	1	0	1
Amortization/curtailment of prior service credit/(cost)	2	2	4
Net actuarial loss arising during the year	15	(8)	(1)
Prior service credit	(33)	0	(8)
Total amount recognized in OCI	(15)	(6)	(4)
Total amount recognized in Comprehensive income	$ (21)	$ (8)	$ (8)