PENSION AND OTHER POSTRETIREMENT BENEFITS - OTHER POSTRETIREMENT BENEFITS (Details) - CAD CAD in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Canada
Change in plan assets
Employer contributions	CAD 14	CAD 0	CAD 0
Presented as follows:
Deferred amounts and other assets	9	8
United States
Change in plan assets
Employer contributions	31	13	15
Presented as follows:
Deferred amounts and other assets	40	44
OPEB | Canada
Change in accumulate postretirement benefit obligation
Benefit obligation at beginning of year	179	173
Service cost	7	4	3
Interest cost	10	6	7
Participant contributions	0	0
Actuarial (gain)/loss	8	(2)
Benefits paid	10	6
Foreign currency exchange rate changes	0	0
Acquired in Merger Transaction	146	0
Other	(3)	0
Benefit obligation at beginning of year	321	179	173
Change in plan assets
Fair value of plan assets at beginning of year	0	0
Actual return on plan assets	0	0
Employer contributions	10	6
Participant contributions	0	0
Benefits paid	10	6
Foreign currency exchange rate changes	0	0
Acquired in Merger Transaction	0	0
Fair value of plan assets at end of year	0	0	0
Underfunded status at end of year	(321)	(179)
Presented as follows:
Deferred amounts and other assets	0	0
Accounts payable and other	(12)	(7)
Other long-term liabilities	309	172
Amount recognized in balance sheet	(321)	(179)
OPEB | United States
Change in accumulate postretirement benefit obligation
Benefit obligation at beginning of year	133	135
Service cost	5	4	5
Interest cost	10	5	4
Participant contributions	4	1
Actuarial (gain)/loss	34	(10)
Benefits paid	19	6
Foreign currency exchange rate changes	(17)	(4)
Acquired in Merger Transaction	254	0
Other	1	(12)
Benefit obligation at beginning of year	337	133	135
Change in plan assets
Fair value of plan assets at beginning of year	115	115
Actual return on plan assets	21	5
Employer contributions	1	3
Participant contributions	4	1
Benefits paid	19	6
Foreign currency exchange rate changes	(11)	(3)
Acquired in Merger Transaction	102	0
Fair value of plan assets at end of year	213	115	CAD 115
Underfunded status at end of year	(124)	(18)
Presented as follows:
Deferred amounts and other assets	7	4
Accounts payable and other	(7)	0
Other long-term liabilities	124	22
Amount recognized in balance sheet	CAD (124)	CAD (18)