PENSION AND OTHER POSTRETIREMENT BENEFITS - PLAN ASSETS (Details) - CAD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Canada \| Equity securities
Pension and Other Postretirement Benefit Disclosures
Allocation of plan assets (as a percent)	43.50%
Plan asset allocations	47.20%	46.40%
Canada \| Fixed income securities
Pension and Other Postretirement Benefit Disclosures
Allocation of plan assets (as a percent)	30.00%
Plan asset allocations	29.60%	31.00%
Canada \| Alternatives
Pension and Other Postretirement Benefit Disclosures
Allocation of plan assets (as a percent)	26.50%
Plan asset allocations	23.20%	22.60%
Canada \| Pension
Change in plan assets
Fair value of plan assets at beginning of year	$ 3,827	$ 3,523
Fair value of plan assets at end of year	4,077	3,827
Canada \| Pension \| Level 1
Change in plan assets
Fair value of plan assets at beginning of year	545
Fair value of plan assets at end of year	600	545
Canada \| Pension \| Level 2
Change in plan assets
Fair value of plan assets at beginning of year	2,430
Fair value of plan assets at end of year	2,565	2,430
Canada \| Pension \| Level 3
Change in plan assets
Fair value of plan assets at beginning of year	852	562
Unrealized and realized gains/(losses)	(27)	10
Purchases and settlements, net	87	280
Fair value of plan assets at end of year	912	852
Canada \| Pension \| Cash and Cash Equivalents
Change in plan assets
Fair value of plan assets at beginning of year	184
Fair value of plan assets at end of year	213	184
Canada \| Pension \| Cash and Cash Equivalents \| Level 1
Change in plan assets
Fair value of plan assets at beginning of year	184
Fair value of plan assets at end of year	213	184
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	348
Fair value of plan assets at end of year	366	348
Canada \| Pension \| Equity Securities, Non-US \| 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	178	165
Canada \| Pension \| Equity Securities, Non-US \| Level 2
Change in plan assets
Fair value of plan assets at beginning of year	183
Fair value of plan assets at end of year	188	183
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	2	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	2	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	1,429
Fair value of plan assets at end of year	1,556	1,429
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	1,429
Fair value of plan assets at end of year	1,556	1,429
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	614
Fair value of plan assets at end of year	585	614
Canada \| Pension \| Government \| Level 1
Change in plan assets
Fair value of plan assets at beginning of year	196
Fair value of plan assets at end of year	207	196
Canada \| Pension \| Government \| Level 2
Change in plan assets
Fair value of plan assets at beginning of year	418
Fair value of plan assets at end of year	378	418
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	388
Fair value of plan assets at end of year	410	388
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	388
Fair value of plan assets at end of year	410	388
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	852
Fair value of plan assets at end of year	912	852
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	852
Fair value of plan assets at end of year	912	852
Canada \| Pension \| Foreign exchange contracts
Change in plan assets
Fair value of plan assets at beginning of year	12
Fair value of plan assets at end of year	33	12
Canada \| Pension \| Foreign exchange 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 \| Foreign exchange contracts \| Level 2
Change in plan assets
Fair value of plan assets at beginning of year	12
Fair value of plan assets at end of year	33	12
Canada \| Pension \| Foreign exchange 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/(losses)	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
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
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
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
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 end of year	0
Canada \| OPEB \| Corporate \| Level 1 \| Fair Value
Change in plan assets
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 end of year	0
Canada \| OPEB \| Corporate \| Level 3 \| Fair Value
Change in plan assets
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	55.60%	55.20%
United States \| Fixed income securities
Pension and Other Postretirement Benefit Disclosures
Allocation of plan assets (as a percent)	20.00%
Plan asset allocations	17.20%	19.80%
United States \| Alternatives
Pension and Other Postretirement Benefit Disclosures
Allocation of plan assets (as a percent)	35.00%
Plan asset allocations	27.20%	25.00%
United States \| Pension
Change in plan assets
Fair value of plan assets at beginning of year	$ 1,104	$ 1,045
Fair value of plan assets at end of year	1,062	1,104
United States \| Pension \| Level 1
Change in plan assets
Fair value of plan assets at beginning of year	14
Fair value of plan assets at end of year	5	14
United States \| Pension \| Level 2
Change in plan assets
Fair value of plan assets at beginning of year	814
Fair value of plan assets at end of year	768	814
United States \| Pension \| Level 3
Change in plan assets
Fair value of plan assets at beginning of year	276	130
Unrealized and realized gains/(losses)	7	13
Purchases and settlements, net	6	133
Fair value of plan assets at end of year	289	276
United States \| Pension \| Cash and Cash Equivalents
Change in plan assets
Fair value of plan assets at beginning of year	14
Fair value of plan assets at end of year	5	14
United States \| Pension \| Cash and Cash Equivalents \| Level 1
Change in plan assets
Fair value of plan assets at beginning of year	14
Fair value of plan assets at end of year	5	14
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	93
Fair value of plan assets at end of year	0	93
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	93
Fair value of plan assets at end of year	0	93
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	516
Fair value of plan assets at end of year	590	516
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	516
Fair value of plan assets at end of year	590	516
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	164
Fair value of plan assets at end of year	75	164
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	164
Fair value of plan assets at end of year	75	164
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	41
Fair value of plan assets at end of year	103	41
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	41
Fair value of plan assets at end of year	103	41
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	276
Fair value of plan assets at end of year	289	276
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	276
Fair value of plan assets at end of year	289	276
United States \| Pension \| Foreign exchange 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 \| Foreign exchange 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 \| Foreign exchange 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 \| Foreign exchange 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	188	181
Fair value of plan assets at end of year	188	188
United States \| OPEB \| Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	188
Fair value of plan assets at end of year	188	188
United States \| OPEB \| Level 1 \| Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	42
Fair value of plan assets at end of year	38	42
United States \| OPEB \| Level 2 \| Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	128
Fair value of plan assets at end of year	128	128
United States \| OPEB \| Level 3
Change in plan assets
Fair value of plan assets at beginning of year	18	5
Unrealized and realized gains/(losses)	1	1
Purchases and settlements, net	3	12
Fair value of plan assets at end of year	22	18
United States \| OPEB \| Level 3 \| Fair Value
Change in plan assets
Fair value of plan assets at beginning of year	18
Fair value of plan assets at end of year	22	18
United States \| OPEB \| Cash and Cash Equivalents \| 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		2
United States \| OPEB \| Cash and Cash Equivalents \| Level 1 \| 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		2
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
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
United States \| OPEB \| Equity Securities, US
Change in plan assets
Fair value of plan assets at beginning of year	75
Fair value of plan assets at end of year	35	75
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	75
Fair value of plan assets at end of year	35	75
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	38
Fair value of plan assets at end of year	79	38
United States \| OPEB \| Equity Securities, Global \| Level 1
Change in plan assets
Fair value of plan assets at beginning of year	0

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

12 Months Ended

Dec. 31, 2020

Dec. 31, 2019

Canada | Equity securities

Pension and Other Postretirement Benefit Disclosures

Allocation of plan assets (as a percent)

43.50%

Plan asset allocations

47.20%

46.40%

Canada | Fixed income securities