PENSION AND OTHER POSTRETIREMENT BENEFITS - NET BENEFIT COSTS RECOGNIZED (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension | Canada
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	$ 149	$ 149	$ 156
Interest cost	139	130	116
Expected return on plan assets	(245)	(245)	(201)
Amortization/settlement of net actuarial loss	41	25	29
Amortization/curtailment of prior service (credit)/cost	0	0	0
Net periodic benefit cost	84	59	100
Defined contribution benefit cost	8	11	11
Net pension cost recognized in Earnings	92	70	111
Amount recognized in OCI:
Effect of plan combination	0	0	0
Amortization/settlement of net actuarial loss	(26)	(11)	(14)
Amortization/curtailment of prior service credit/(cost)	0	0	0
Net actuarial loss arising during the year	115	112	38
Total amount recognized in OCI	89	101	24
Total amount recognized in Comprehensive income	181	171	135
Expected amortization, next fiscal year	21
Pension | United States
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	45	45	48
Interest cost	41	38	35
Expected return on plan assets	(78)	(88)	(57)
Amortization/settlement of net actuarial loss	2	7	10
Amortization/curtailment of prior service (credit)/cost	(1)	3	0
Net periodic benefit cost	9	5	36
Defined contribution benefit cost	0	0	0
Net pension cost recognized in Earnings	9	5	36
Amount recognized in OCI:
Effect of plan combination	(6)	0	0
Amortization/settlement of net actuarial loss	(2)	(7)	(9)
Amortization/curtailment of prior service credit/(cost)	1	(3)	0
Net actuarial loss arising during the year	8	28	0
Total amount recognized in OCI	1	18	(9)
Total amount recognized in Comprehensive income	10	23	27
Expected amortization, next fiscal year	0
OPEB | Canada
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	5	8	7
Interest cost	10	10	10
Expected return on plan assets	0	0	0
Amortization/settlement of net actuarial loss	(7)	0	0
Amortization/curtailment of prior service (credit)/cost	(1)	0	1
Net pension cost recognized in Earnings	7	18	18
Amount recognized in OCI:
Amortization/settlement of net actuarial loss	7	0	(1)
Amortization/curtailment of prior service credit/(cost)	1	0	0
Net actuarial loss arising during the year	15	(46)	(8)
Prior service (credit)/cost	0	0	(3)
Total amount recognized in OCI	23	(46)	(12)
Total amount recognized in Comprehensive income	30	(28)	6
Expected amortization, next fiscal year	1
OPEB | United States
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	2	3	5
Interest cost	10	10	10
Expected return on plan assets	(12)	(12)	(10)
Amortization/settlement of net actuarial loss	0	(1)	0
Amortization/curtailment of prior service (credit)/cost	(2)	(4)	0
Net pension cost recognized in Earnings	(2)	(4)	5
Amount recognized in OCI:
Amortization/settlement of net actuarial loss	0	1	1
Amortization/curtailment of prior service credit/(cost)	2	4	0
Net actuarial loss arising during the year	(8)	(1)	(42)
Prior service (credit)/cost	0	(8)	1
Total amount recognized in OCI	(6)	(4)	(40)
Total amount recognized in Comprehensive income	(8)	$ (8)	$ (35)
Expected amortization, next fiscal year	$ 3