PENSION AND OTHER POSTRETIREMENT BENEFITS - BENEFIT OBLIGATION, PLAN ASSETS AND FUNDED STATUS (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
Canada | OPEB
Change in projected benefit obligation
Benefit obligation at beginning of year	179	173
Service cost	7	4	3
Interest cost	10	6	7
Actuarial 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
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)
Canada | Pension
Change in projected benefit obligation
Benefit obligation at beginning of year	2,270	2,064
Service cost	156	129	137
Interest cost	116	73	81
Actuarial loss	145	97
Benefits paid	(165)	(87)
Foreign currency exchange rate changes	0	0
Acquired in Merger Transaction	1,505	0
Plan settlements	0	0
Other	6	(6)
Benefit obligation at beginning of year	4,033	2,270	2,064
Change in plan assets
Fair value of plan assets at beginning of year	2,019	1,886
Actual return on plan assets	308	146
Employer contributions	161	74
Benefits paid	(165)	(87)
Foreign currency exchange rate changes	0	0
Acquired in Merger Transaction	1,290	0
Plan settlements	0	0
Other	6	0
Fair value of plan assets at end of year	3,619	2,019	1,886
Underfunded status at end of year	(414)	(251)
Presented as follows:
Deferred amounts and other assets	38	5
Accounts payable and other	(60)	0
Other long-term liabilities	(392)	(256)
Amount recognized in balance sheet	(414)	(251)
Accumulated benefit obligation	3,700	978
United States
Change in plan assets
Employer contributions	31	13	15
Presented as follows:
Deferred amounts and other assets	40	44
United States | OPEB
Change in projected benefit obligation
Benefit obligation at beginning of year	133	135
Service cost	5	4	5
Interest cost	10	5	4
Actuarial 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
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	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	(124)	(18)
United States | Pension
Change in projected benefit obligation
Benefit obligation at beginning of year	508	487
Service cost	48	26	30
Interest cost	35	16	17
Actuarial loss	57	15
Benefits paid	(42)	(21)
Foreign currency exchange rate changes	(63)	(14)
Acquired in Merger Transaction	811	0
Plan settlements	(59)	0
Other	(16)	(1)
Benefit obligation at beginning of year	1,279	508	487
Change in plan assets
Fair value of plan assets at beginning of year	361	343
Actual return on plan assets	113	22
Employer contributions	57	28
Benefits paid	(42)	(21)
Foreign currency exchange rate changes	(51)	(10)
Acquired in Merger Transaction	731	0
Plan settlements	(59)	0
Other	(13)	(1)
Fair value of plan assets at end of year	1,097	361	CAD 343
Underfunded status at end of year	(182)	(147)
Presented as follows:
Deferred amounts and other assets	0	0
Accounts payable and other	(3)	0
Other long-term liabilities	(179)	(147)
Amount recognized in balance sheet	(182)	(147)
Accumulated benefit obligation	CAD 1,200	CAD 462