PENSION AND OTHER POSTRETIREMENT BENEFITS - PLAN ASSETS (Details) - CAD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Canada \| Equity securities
Pension and Other Postretirement Benefit Disclosures
Allocation of plan assets (as a percent)	43.40%
Plan asset allocations	46.40%	45.80%
Canada \| Fixed income securities
Pension and Other Postretirement Benefit Disclosures
Allocation of plan assets (as a percent)	30.30%
Plan asset allocations	31.00%	38.80%
Canada \| Alternatives
Pension and Other Postretirement Benefit Disclosures
Allocation of plan assets (as a percent)	26.30%
Plan asset allocations	22.60%	15.40%
Canada \| Pension
Change in plan assets
Fair value of plan assets at beginning of year	$ 3,523	$ 3,619
Fair value of plan assets at end of year	3,827	3,523
Canada \| Pension \| Level 1
Change in plan assets
Fair value of plan assets at beginning of year	596
Fair value of plan assets at end of year	545	596
Canada \| Pension \| Level 2
Change in plan assets
Fair value of plan assets at beginning of year	2,365
Fair value of plan assets at end of year	2,430	2,365
Canada \| Pension \| Level 3
Change in plan assets
Fair value of plan assets at beginning of year	562	340
Unrealized and realized gains	10	77
Purchases and settlements, net	280	145
Fair value of plan assets at end of year	852	562
Canada \| Pension \| Cash and Cash Equivalents
Change in plan assets
Fair value of plan assets at beginning of year	240
Fair value of plan assets at end of year	184	240
Canada \| Pension \| Cash and Cash Equivalents \| Level 1
Change in plan assets
Fair value of plan assets at beginning of year	240
Fair value of plan assets at end of year	184	240
Canada \| Pension \| Cash and Cash Equivalents \| 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 \| Pension \| Cash and Cash Equivalents \| 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, Non-US
Change in plan assets
Fair value of plan assets at beginning of year	619
Fair value of plan assets at end of year	348	619
Canada \| Pension \| Equity Securities, Non-US \| Level 1
Change in plan assets
Fair value of plan assets at beginning of year	138
Fair value of plan assets at end of year	165	138
Canada \| Pension \| Equity Securities, Non-US \| Level 2
Change in plan assets
Fair value of plan assets at beginning of year	481
Fair value of plan assets at end of year	183	481
Canada \| Pension \| Equity Securities, Non-US \| 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, US
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, US \| 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 \| Pension \| Equity Securities, US \| 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 \| Pension \| Equity Securities, US \| 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	992
Fair value of plan assets at end of year	1,429	992
Canada \| Pension \| 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 \| Pension \| Equity Securities, Global \| Level 2
Change in plan assets
Fair value of plan assets at beginning of year	992
Fair value of plan assets at end of year	1,429	992
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 \| Government
Change in plan assets
Fair value of plan assets at beginning of year	671
Fair value of plan assets at end of year	614	671
Canada \| Pension \| Government \| Level 1
Change in plan assets
Fair value of plan assets at beginning of year	218
Fair value of plan assets at end of year	196	218
Canada \| Pension \| Government \| Level 2
Change in plan assets
Fair value of plan assets at beginning of year	453
Fair value of plan assets at end of year	418	453
Canada \| Pension \| Government \| 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 \| Corporate
Change in plan assets
Fair value of plan assets at beginning of year	457
Fair value of plan assets at end of year	388	457
Canada \| Pension \| Corporate \| 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 \| Pension \| Corporate \| Level 2
Change in plan assets
Fair value of plan assets at beginning of year	457
Fair value of plan assets at end of year	388	457
Canada \| Pension \| Corporate \| 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 \| Infrastructure and real estate
Change in plan assets
Fair value of plan assets at beginning of year	562
Fair value of plan assets at end of year	852	562
Canada \| Pension \| Infrastructure and real estate \| 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 \| Pension \| Infrastructure and real estate \| 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 \| Pension \| Infrastructure and real estate \| Level 3
Change in plan assets
Fair value of plan assets at beginning of year	562
Fair value of plan assets at end of year	852	562
Canada \| Pension \| Forward currency contracts
Change in plan assets
Fair value of plan assets at beginning of year	(18)
Fair value of plan assets at end of year	12	(18)
Canada \| Pension \| Forward currency contracts \| 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 \| Pension \| Forward currency contracts \| Level 2
Change in plan assets
Fair value of plan assets at beginning of year	(18)
Fair value of plan assets at end of year	12	(18)
Canada \| Pension \| Forward currency contracts \| 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
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
Change in plan assets
Fair value of plan assets at beginning of year	0	0
Unrealized and realized gains	0	0
Purchases and settlements, net	0	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 \| 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 \| Cash and Cash Equivalents \| 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 \| Cash and Cash Equivalents \| 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, US
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, US \| 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, US \| 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, US \| 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 \| 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
Canada \| OPEB \| 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 \| OPEB \| 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 \| OPEB \| Corporate \| 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 \| OPEB \| Corporate \| 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
Canada \| OPEB \| 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		0
Canada \| OPEB \| 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
Canada \| OPEB \| Infrastructure and real estate \| 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 \| 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 \| OPEB \| 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 \| OPEB \| Infrastructure and real estate \| 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 \| Equity securities
Pension and Other Postretirement Benefit Disclosures
Allocation of plan assets (as a percent)	45.00%
Plan asset allocations	55.20%	52.70%
United States \| Fixed income securities
Pension and Other Postretirement Benefit Disclosures
Allocation of plan assets (as a percent)	20.00%
Plan asset allocations	19.80%	34.90%
United States \| Alternatives
Pension and Other Postretirement Benefit Disclosures
Allocation of plan assets (as a percent)	35.00%
Plan asset allocations	25.00%	12.40%
United States \| Pension
Change in plan assets
Fair value of plan assets at beginning of year	$ 1,045	$ 1,097
Fair value of plan assets at end of year	1,104	1,045
United States \| Pension \| Level 1
Change in plan assets
Fair value of plan assets at beginning of year	56
Fair value of plan assets at end of year	14	56
United States \| Pension \| Level 2
Change in plan assets
Fair value of plan assets at beginning of year	859
Fair value of plan assets at end of year	814	859
United States \| Pension \| Level 3
Change in plan assets
Fair value of plan assets at beginning of year	130	56
Unrealized and realized gains	13	9
Purchases and settlements, net	133	65
Fair value of plan assets at end of year	276	130
United States \| Pension \| Cash and Cash Equivalents
Change in plan assets
Fair value of plan assets at beginning of year	56
Fair value of plan assets at end of year	14	56
United States \| Pension \| Cash and Cash Equivalents \| Level 1
Change in plan assets
Fair value of plan assets at beginning of year	56
Fair value of plan assets at end of year	14	56
United States \| Pension \| Cash and Cash Equivalents \| 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 \| Cash and Cash Equivalents \| 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, Non-US
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, Non-US \| 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, Non-US \| 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, Non-US \| 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, US
Change in plan assets
Fair value of plan assets at beginning of year	110
Fair value of plan assets at end of year	93	110
United States \| Pension \| Equity Securities, US \| 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, US \| Level 2
Change in plan assets
Fair value of plan assets at beginning of year	110
Fair value of plan assets at end of year	93	110
United States \| Pension \| Equity Securities, US \| 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	440
Fair value of plan assets at end of year	516	440
United States \| Pension \| 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
United States \| Pension \| Equity Securities, Global \| Level 2
Change in plan assets
Fair value of plan assets at beginning of year	440
Fair value of plan assets at end of year	516	440
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 \| Government
Change in plan assets
Fair value of plan assets at beginning of year	265
Fair value of plan assets at end of year	164	265
United States \| Pension \| Government \| 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 \| Government \| Level 2
Change in plan assets
Fair value of plan assets at beginning of year	265
Fair value of plan assets at end of year	164	265
United States \| Pension \| Government \| 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 \| Corporate
Change in plan assets
Fair value of plan assets at beginning of year	44
Fair value of plan assets at end of year	41	44
United States \| Pension \| Corporate \| 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 \| Corporate \| Level 2
Change in plan assets
Fair value of plan assets at beginning of year	44
Fair value of plan assets at end of year	41	44
United States \| Pension \| Corporate \| 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 \| Infrastructure and real estate
Change in plan assets
Fair value of plan assets at beginning of year	130
Fair value of plan assets at end of year	276	130
United States \| Pension \| Infrastructure and real estate \| 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 \| Infrastructure and real estate \| 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 \| Infrastructure and real estate \| Level 3
Change in plan assets
Fair value of plan assets at beginning of year	130
Fair value of plan assets at end of year	276	130
United States \| Pension \| Forward currency contracts
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 \| Forward currency contracts \| 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 \| Forward currency contracts \| 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 \| Forward currency contracts \| 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
Change in plan assets
Fair value of plan assets at beginning of year	181	213
Fair value of plan assets at end of year	188	181
United States \| OPEB \| Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	181
Fair value of plan assets at end of year	188	181
United States \| OPEB \| Level 1 \| Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	50
Fair value of plan assets at end of year	42	50
United States \| OPEB \| Level 2 \| Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	126
Fair value of plan assets at end of year	128	126
United States \| OPEB \| Level 3
Change in plan assets
Fair value of plan assets at beginning of year	5	0
Unrealized and realized gains	1	0
Purchases and settlements, net	12	5
Fair value of plan assets at end of year	18	5
United States \| OPEB \| Level 3 \| Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	5
Fair value of plan assets at end of year	18	5
United States \| OPEB \| Cash and Cash Equivalents \| Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	7
Fair value of plan assets at end of year	2	7
United States \| OPEB \| Cash and Cash Equivalents \| Level 1 \| Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	7
Fair value of plan assets at end of year	2	7
United States \| OPEB \| Cash and Cash Equivalents \| 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 \| Cash and Cash Equivalents \| 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, US
Change in plan assets
Fair value of plan assets at beginning of year	68
Fair value of plan assets at end of year	75	68
United States \| OPEB \| Equity Securities, US \| 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 \| OPEB \| Equity Securities, US \| Level 2
Change in plan assets
Fair value of plan assets at beginning of year	68
Fair value of plan assets at end of year	75	68
United States \| OPEB \| Equity Securities, US \| 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	30

PENSION AND OTHER POSTRETIREMENT BENEFITS - PLAN ASSETS (Details) - CAD ($)
$ in Millions

12 Months Ended

Dec. 31, 2019

Dec. 31, 2018

Canada | Equity securities

Pension and Other Postretirement Benefit Disclosures

Allocation of plan assets (as a percent)

43.40%

Plan asset allocations

46.40%

45.80%

Canada | Fixed income securities