PENSION AND OTHER POSTRETIREMENT BENEFITS - PLAN ASSETS (Details) - CAD CAD in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Pension | Equity securities | United States | Level 2
Change in plan assets
Fair value of plan assets at end of year	CAD 0
Pension | Equity securities | United States | Level 3
Change in plan assets
Fair value of plan assets at end of year	0
Canada | Level 3
Change in plan assets
Fair value of plan assets at beginning of year	281	CAD 248
Unrealized and realized gains	26	20
Purchases and settlements, net	33	13
Fair value of plan assets at end of year	CAD 340	CAD 281
Canada | Equity securities
Pension and Other Postretirement Benefit Disclosures
Plan asset allocations	52.00%	47.00%
Canada | Fixed income securities
Pension and Other Postretirement Benefit Disclosures
Plan asset allocations	34.20%	39.00%
Canada | Other
Pension and Other Postretirement Benefit Disclosures
Plan asset allocations	13.80%	14.00%
Canada | Pension
Change in plan assets
Fair value of plan assets at beginning of year	CAD 2,019	CAD 1,886
Fair value of plan assets at end of year	3,619	2,019
Canada | Pension | Level 1
Change in plan assets
Fair value of plan assets at beginning of year	1,593
Fair value of plan assets at end of year	2,861	1,593
Canada | Pension | Level 2
Change in plan assets
Fair value of plan assets at beginning of year	145
Fair value of plan assets at end of year	418	145
Canada | Pension | Level 3
Change in plan assets
Fair value of plan assets at beginning of year	281
Fair value of plan assets at end of year	340	281
Canada | Pension | Level 3 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year		0
Canada | Pension | Equity securities | United States
Change in plan assets
Fair value of plan assets at beginning of year	219
Fair value of plan assets at end of year	427	219
Canada | Pension | Equity securities | United States | Level 1
Change in plan assets
Fair value of plan assets at beginning of year	219
Fair value of plan assets at end of year	427	219
Canada | Pension | Equity securities | United States | Level 2
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year		0
Canada | Pension | Equity securities | United States | Level 3
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year		0
Canada | Pension | Equity securities | Canada
Change in plan assets
Fair value of plan assets at beginning of year	425
Fair value of plan assets at end of year	1,267	425
Canada | Pension | Equity securities | Canada | Level 1
Change in plan assets
Fair value of plan assets at beginning of year	425
Fair value of plan assets at end of year	842	425
Canada | Pension | Equity securities | Canada | Level 2
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	425	0
Canada | Pension | Equity securities | Canada | Level 3
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Canada | Pension | Equity securities | Global
Change in plan assets
Fair value of plan assets at beginning of year	305
Fair value of plan assets at end of year	189	305
Canada | Pension | Equity securities | Global | Level 1
Change in plan assets
Fair value of plan assets at beginning of year	165
Fair value of plan assets at end of year	189	165
Canada | Pension | Equity securities | Global | Level 2
Change in plan assets
Fair value of plan assets at beginning of year	140
Fair value of plan assets at end of year	0	140
Canada | Pension | Equity securities | Global | Level 3
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Canada | Pension | Cash and cash equivalents | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	156
Fair value of plan assets at end of year	169	156
Canada | Pension | Cash and cash equivalents | Level 1 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	156
Fair value of plan assets at end of year	169	156
Canada | Pension | Government | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	351
Fair value of plan assets at end of year	933	351
Canada | Pension | Government | Level 1 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	351
Fair value of plan assets at end of year	933	351
Canada | Pension | Government | Level 2 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Canada | Pension | Government | Level 3 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Canada | Pension | Corporate | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	280
Fair value of plan assets at end of year	304	280
Canada | Pension | Corporate | Level 1 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	277
Fair value of plan assets at end of year	301	277
Canada | Pension | Corporate | Level 2 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	3
Fair value of plan assets at end of year	3	3
Canada | Pension | Corporate | Level 3 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Canada | Pension | Infrastructure and real estate | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	281
Fair value of plan assets at end of year	340	281
Canada | Pension | Infrastructure and real estate | Level 1 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Canada | Pension | Infrastructure and real estate | Level 2 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Canada | Pension | Infrastructure and real estate | Level 3 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	281
Fair value of plan assets at end of year	340	281
Canada | Pension | Forward currency contracts | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	2
Fair value of plan assets at end of year	(10)	2
Canada | Pension | Forward currency contracts | Level 1 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Canada | Pension | Forward currency contracts | Level 2 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	2
Fair value of plan assets at end of year	(10)	2
Canada | Pension | Forward currency contracts | Level 3 | Fair Value
Change in plan assets
Fair value of plan assets at end of year	0
Canada | OPEB
Change in plan assets
Fair value of plan assets at beginning of year	0	0
Fair value of plan assets at end of year	0	0
Canada | OPEB | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Canada | OPEB | Level 1 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Canada | OPEB | Level 2 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Canada | OPEB | Level 3 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Canada | OPEB | Equity securities | United States
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Canada | OPEB | Equity securities | United States | Level 1
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Canada | OPEB | Equity securities | United States | Level 2
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Canada | OPEB | Equity securities | United States | Level 3
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Canada | OPEB | Equity securities | Global
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Canada | OPEB | Equity securities | Global | Level 1
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Canada | OPEB | Equity securities | Global | Level 2
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Canada | OPEB | Equity securities | Global | Level 3
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Canada | OPEB | Cash and cash equivalents | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Canada | OPEB | Cash and cash equivalents | Level 1 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Canada | OPEB | Government | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Canada | OPEB | Government | Level 1 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
United States | Level 3
Change in plan assets
Fair value of plan assets at beginning of year	40	49
Unrealized and realized gains	5	2
Purchases and settlements, net	11	(11)
Fair value of plan assets at end of year	CAD 56	CAD 40
United States | Equity securities
Pension and Other Postretirement Benefit Disclosures
Plan asset allocations	47.10%	55.40%
United States | Fixed income securities
Pension and Other Postretirement Benefit Disclosures
Plan asset allocations	47.70%	33.00%
United States | Other
Pension and Other Postretirement Benefit Disclosures
Plan asset allocations	5.20%	11.60%
United States | Pension
Change in plan assets
Fair value of plan assets at beginning of year	CAD 361	CAD 343
Fair value of plan assets at end of year	1,097	361
United States | Pension | Level 1
Change in plan assets
Fair value of plan assets at beginning of year	289
Fair value of plan assets at end of year	989	289
United States | Pension | Level 1 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year		0
United States | Pension | Level 2
Change in plan assets
Fair value of plan assets at beginning of year	32
Fair value of plan assets at end of year	52	32
United States | Pension | Level 3
Change in plan assets
Fair value of plan assets at beginning of year	40
Fair value of plan assets at end of year	56	40
United States | Pension | Level 3 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year		0
United States | Pension | Equity securities
Change in plan assets
Fair value of plan assets at end of year	343
United States | Pension | Equity securities | United States
Change in plan assets
Fair value of plan assets at beginning of year	54
Fair value of plan assets at end of year		54
United States | Pension | Equity securities | United States | Level 1
Change in plan assets
Fair value of plan assets at beginning of year	54
Fair value of plan assets at end of year	343	54
United States | Pension | Equity securities | United States | Level 2
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year		0
United States | Pension | Equity securities | United States | Level 3
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year		0
United States | Pension | Equity securities | Canada
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
United States | Pension | Equity securities | Canada | Level 1
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
United States | Pension | Equity securities | Canada | Level 2
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
United States | Pension | Equity securities | Canada | Level 3
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
United States | Pension | Equity securities | Global
Change in plan assets
Fair value of plan assets at beginning of year	146
Fair value of plan assets at end of year	174	146
United States | Pension | Equity securities | Global | Level 1
Change in plan assets
Fair value of plan assets at beginning of year	116
Fair value of plan assets at end of year	122	116
United States | Pension | Equity securities | Global | Level 2
Change in plan assets
Fair value of plan assets at beginning of year	30
Fair value of plan assets at end of year	52	30
United States | Pension | Equity securities | Global | Level 3
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
United States | Pension | Cash and cash equivalents | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	3
Fair value of plan assets at end of year	2	3
United States | Pension | Cash and cash equivalents | Level 1 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	3
Fair value of plan assets at end of year	2	3
United States | Pension | Government | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
United States | Pension | Government | Level 1 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
United States | Pension | Government | Level 2 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
United States | Pension | Government | Level 3 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
United States | Pension | Corporate | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	116
Fair value of plan assets at end of year	523	116
United States | Pension | Corporate | Level 1 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	116
Fair value of plan assets at end of year	522	116
United States | Pension | Corporate | Level 2 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	1	0
United States | Pension | Corporate | Level 3 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
United States | Pension | Infrastructure and real estate | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	40
Fair value of plan assets at end of year	56	40
United States | Pension | Infrastructure and real estate | Level 1 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
United States | Pension | Infrastructure and real estate | Level 2 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
United States | Pension | Infrastructure and real estate | Level 3 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	40
Fair value of plan assets at end of year	56	40
United States | Pension | Forward currency contracts | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	2
Fair value of plan assets at end of year	(1)	2
United States | Pension | Forward currency contracts | Level 1 | Fair Value
Change in plan assets
Fair value of plan assets at end of year	0
United States | Pension | Forward currency contracts | Level 2 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	2
Fair value of plan assets at end of year	(1)	2
United States | Pension | Forward currency contracts | Level 3 | Fair Value
Change in plan assets
Fair value of plan assets at end of year	0
United States | OPEB
Change in plan assets
Fair value of plan assets at beginning of year	115	115
Fair value of plan assets at end of year	213	115
United States | OPEB | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	115
Fair value of plan assets at end of year	213	115
United States | OPEB | Level 1 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	115
Fair value of plan assets at end of year	213	115
United States | OPEB | Level 2 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
United States | OPEB | Level 3 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
United States | OPEB | Equity securities | United States
Change in plan assets
Fair value of plan assets at beginning of year	35
Fair value of plan assets at end of year	80	35
United States | OPEB | Equity securities | United States | Level 1
Change in plan assets
Fair value of plan assets at beginning of year	35
Fair value of plan assets at end of year	80	35
United States | OPEB | Equity securities | United States | Level 2
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
United States | OPEB | Equity securities | United States | Level 3
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
United States | OPEB | Equity securities | Global
Change in plan assets
Fair value of plan assets at beginning of year	34
Fair value of plan assets at end of year	36	34
United States | OPEB | Equity securities | Global | Level 1
Change in plan assets
Fair value of plan assets at beginning of year	34
Fair value of plan assets at end of year	36	34
United States | OPEB | Equity securities | Global | Level 2
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
United States | OPEB | Equity securities | Global | Level 3
Change in plan assets
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
United States | OPEB | Cash and cash equivalents | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	1
Fair value of plan assets at end of year	1	1
United States | OPEB | Cash and cash equivalents | Level 1 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	1
Fair value of plan assets at end of year	1	1
United States | OPEB | Government | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	45
Fair value of plan assets at end of year	96	45
United States | OPEB | Government | Level 1 | Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	45
Fair value of plan assets at end of year	CAD 96	CAD 45
Minimum | Canada | Equity securities
Pension and Other Postretirement Benefit Disclosures
Allocation of plan assets (as a percent)	40.00%
Minimum | Canada | Fixed income securities
Pension and Other Postretirement Benefit Disclosures
Allocation of plan assets (as a percent)	27.50%
Minimum | Canada | Other
Pension and Other Postretirement Benefit Disclosures
Allocation of plan assets (as a percent)	0.00%
Minimum | United States | Equity securities
Pension and Other Postretirement Benefit Disclosures
Allocation of plan assets (as a percent)	52.50%
Minimum | United States | Fixed income securities
Pension and Other Postretirement Benefit Disclosures
Allocation of plan assets (as a percent)	27.50%
Minimum | United States | Other
Pension and Other Postretirement Benefit Disclosures
Allocation of plan assets (as a percent)	0.00%
Maximum | Canada | Equity securities
Pension and Other Postretirement Benefit Disclosures
Allocation of plan assets (as a percent)	70.00%
Maximum | Canada | Fixed income securities
Pension and Other Postretirement Benefit Disclosures
Allocation of plan assets (as a percent)	60.00%
Maximum | Canada | Other
Pension and Other Postretirement Benefit Disclosures
Allocation of plan assets (as a percent)	20.00%
Maximum | United States | Equity securities
Pension and Other Postretirement Benefit Disclosures
Allocation of plan assets (as a percent)	70.00%
Maximum | United States | Fixed income securities
Pension and Other Postretirement Benefit Disclosures
Allocation of plan assets (as a percent)	30.00%
Maximum | United States | Other
Pension and Other Postretirement Benefit Disclosures
Allocation of plan assets (as a percent)	20.00%