PENSION AND OTHER POSTRETIREMENT BENEFITS - BENEFIT OBLIGATION, PLAN ASSETS AND FUNDED STATUS (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Presented as follows:
Accumulated benefit obligation		$ 1,200
Canada | Pension
Change in projected benefit obligation
Accumulated postretirement benefit obligation at beginning of year	$ 4,855	4,446
Service cost	139	148	$ 149
Interest cost	101	128	139
Participant contributions	28	31
Actuarial (gain)/loss	(329)	292
Benefits paid	(194)	(190)
Foreign currency exchange rate changes	0	0
Other	0	0
Accumulated postretirement benefit obligation at end of year	4,600	4,855	4,446
Change in plan assets
Fair value of plan assets at beginning of year	4,077	3,827
Actual return on plan assets	505	288
Employer contributions	120	121
Participant contributions	28	31
Benefits paid	(194)	(190)
Foreign currency exchange rate changes	0	0
Other	0	0
Fair value of plan assets at end of year	4,536	4,077	3,827
Overfunded/(underfunded) status at end of year	(64)	(778)
Presented as follows:
Deferred amounts and other assets	250	35
Accounts payable and other	(9)	(9)
Other long-term liabilities	(305)	(804)
Amount recognized in balance sheet	(64)	(778)
Accumulated benefit obligation	4,300	4,500
Canada | Supplemental Employee Retirement Plan
Presented as follows:
Deferred amounts and other assets	13	11
Canada | OPEB
Change in projected benefit obligation
Accumulated postretirement benefit obligation at beginning of year	321	293
Service cost	6	5	5
Interest cost	7	8	10
Participant contributions	0	0
Actuarial (gain)/loss	(51)	21
Benefits paid	(9)	(6)
Plan amendments	0	0
Foreign currency exchange rate changes	0	0
Other	0	0
Accumulated postretirement benefit obligation at end of year	274	321	293
Change in plan assets
Fair value of plan assets at beginning of year	0	0
Actual return on plan assets	0	0
Employer contributions	9	6
Participant contributions	0	0
Benefits paid	(9)	(6)
Foreign currency exchange rate changes	0	0
Other	0	0
Fair value of plan assets at end of year	0	0	0
Overfunded/(underfunded) status at end of year	(274)	(321)
Presented as follows:
Deferred amounts and other assets	0	0
Accounts payable and other	(12)	(13)
Other long-term liabilities	(262)	(308)
Amount recognized in balance sheet	(274)	(321)
United States
Change in plan assets
Employer contributions	27	27	27
United States | Pension
Change in projected benefit obligation
Accumulated postretirement benefit obligation at beginning of year	1,243	1,230
Service cost	44	44	45
Interest cost	17	31	41
Participant contributions	0	0
Actuarial (gain)/loss	(21)	95
Benefits paid	(84)	(128)
Foreign currency exchange rate changes	(11)	(23)
Other	(4)	(6)
Accumulated postretirement benefit obligation at end of year	1,184	1,243	1,230
Change in plan assets
Fair value of plan assets at beginning of year	1,062	1,104
Actual return on plan assets	151	83
Employer contributions	43	27
Participant contributions	0	0
Benefits paid	(84)	(128)
Foreign currency exchange rate changes	(8)	(18)
Other	(4)	(6)
Fair value of plan assets at end of year	1,160	1,062	1,104
Overfunded/(underfunded) status at end of year	(24)	(181)
Presented as follows:
Deferred amounts and other assets	98	0
Accounts payable and other	(4)	(3)
Other long-term liabilities	(118)	(178)
Amount recognized in balance sheet	(24)	(181)
Accumulated benefit obligation	1,100
United States | Supplemental Employee Retirement Plan
Presented as follows:
Deferred amounts and other assets	84	59
United States | OPEB
Change in projected benefit obligation
Accumulated postretirement benefit obligation at beginning of year	254	288
Service cost	1	2	2
Interest cost	3	7	10
Participant contributions	8	4
Actuarial (gain)/loss	(69)	17
Benefits paid	(22)	(28)
Plan amendments	0	(33)
Foreign currency exchange rate changes	(3)	(4)
Other	1	1
Accumulated postretirement benefit obligation at end of year	173	254	288
Change in plan assets
Fair value of plan assets at beginning of year	188	188
Actual return on plan assets	22	14
Employer contributions	6	12
Participant contributions	8	4
Benefits paid	(22)	(28)
Foreign currency exchange rate changes	(3)	(3)
Other	2	1
Fair value of plan assets at end of year	201	188	$ 188
Overfunded/(underfunded) status at end of year	28	(66)
Presented as follows:
Deferred amounts and other assets	71	19
Accounts payable and other	0	(6)
Other long-term liabilities	(43)	(79)
Amount recognized in balance sheet	$ 28	$ (66)