PENSION AND OTHER POSTRETIREMENT BENEFITS - NET BENEFIT COSTS RECOGNIZED (Details) - CAD CAD in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension | Canada
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	CAD 156	CAD 129	CAD 137
Interest cost	116	73	81
Expected return on plan assets	(201)	(127)	(120)
Amortization of actuarial loss	29	32	39
Net defined benefit costs	100	107	137
Defined contribution benefit costs	11	3	3
Net benefit cost recognized in Earnings	111	110	140
Amount recognized in OCI:
Net actuarial (gain)/loss arising during the year	38	28	(58)
Amortization of net actuarial gain	(14)	(14)	(20)
Total amount recognized in OCI	24	14	(78)
Total amount recognized in Comprehensive income	135	124	62
Expected amortization, next fiscal year	25
Pension | United States
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	48	26	30
Interest cost	35	16	17
Expected return on plan assets	(57)	(21)	(22)
Amortization of actuarial loss	10	3	10
Net defined benefit costs	36	24	35
Defined contribution benefit costs	15	0	0
Net benefit cost recognized in Earnings	51	24	35
Amount recognized in OCI:
Net actuarial (gain)/loss arising during the year	0	16	(19)
Amortization of net actuarial gain	(9)	(6)	(10)
Total amount recognized in OCI	(9)	10	(29)
Total amount recognized in Comprehensive income	42	34	6
Expected amortization, next fiscal year	4
OPEB | Canada
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	7	4	3
Interest cost	10	6	7
Expected return on plan assets	0	0	0
Amortization of actuarial loss	1	0	1
Net defined benefit costs	18	10	11
Amount recognized in OCI:
Net actuarial (gain)/loss arising during the year	(8)	2	2
Amortization of net actuarial gain	(1)	(1)	(1)
Prior service cost	(3)	0	0
Total amount recognized in OCI	(12)	1	1
Total amount recognized in Comprehensive income	6	11	12
Expected amortization, next fiscal year	0
OPEB | United States
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	5	4	5
Interest cost	10	5	4
Expected return on plan assets	(10)	(6)	(6)
Amortization of actuarial loss	0	0	0
Net defined benefit costs	5	3	3
Amount recognized in OCI:
Net actuarial (gain)/loss arising during the year	(42)	12	16
Amortization of net actuarial gain	1	(1)	0
Prior service cost	1	(12)	(7)
Total amount recognized in OCI	(40)	(1)	9
Total amount recognized in Comprehensive income	(35)	CAD 2	CAD 12
Expected amortization, next fiscal year	CAD 2