PENSION AND OTHER POSTRETIREMENT BENEFITS - Benefit Obligation, Plan Assets and Funded Status (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Change in plan assets
Fair value of plan assets at beginning of year	$ 32
Fair value of plan assets at end of year	61	$ 32
Canada | Pension
Change in projected benefit obligation
Projected benefit obligation at beginning of year	3,630	4,600
Service cost	81	131	$ 139
Interest cost	184	127	101
Participant contributions	31	29
Actuarial gain	359	(1,069)
Benefits paid	(193)	(187)
Foreign currency exchange rate changes	0	0
Other	0	(1)
Projected benefit obligation at end of year	4,092	3,630	4,600
Change in plan assets
Fair value of plan assets at beginning of year	4,234	4,536
Actual return/(loss) on plan assets	427	(235)
Employer contributions	27	91
Participant contributions	31	29
Benefits paid	(193)	(187)
Foreign currency exchange rate changes	0	0
Other	2	0
Fair value of plan assets at end of year	4,528	4,234	4,536
Overfunded status at end of year	436	604
Presented as follows:
Deferred amounts and other assets	636	764
Other current liabilities	(8)	(9)
Other long-term liabilities	(192)	(151)
Amount recognized in balance sheet	436	604
Accumulated benefit obligation	3,800	3,400
Canada | Supplemental Employee Retirement Plan
Presented as follows:
Deferred amounts and other assets	14	10
Canada | OPEB
Change in projected benefit obligation
Projected benefit obligation at beginning of year	211	274
Service cost	3	4	6
Interest cost	11	7	7
Participant contributions	0	0
Actuarial gain	13	(66)
Benefits paid	(10)	(8)
Foreign currency exchange rate changes	0	0
Projected benefit obligation at end of year	228	211	274
Change in plan assets
Fair value of plan assets at beginning of year	0	0
Actual return/(loss) on plan assets	0	0
Employer contributions	10	8
Participant contributions	0	0
Benefits paid	(10)	(8)
Foreign currency exchange rate changes	0	0
Fair value of plan assets at end of year	0	0	0
Overfunded status at end of year	(228)	(211)
Presented as follows:
Deferred amounts and other assets	0	0
Other current liabilities	(12)	(12)
Other long-term liabilities	(216)	(199)
Amount recognized in balance sheet	(228)	(211)
US
Change in plan assets
Employer contributions	33	30	27
US | Pension
Change in projected benefit obligation
Projected benefit obligation at beginning of year	1,029	1,184
Service cost	40	43	44
Interest cost	47	24	17
Participant contributions	0	0
Actuarial gain	31	(201)
Benefits paid	(76)	(94)
Foreign currency exchange rate changes	(29)	77
Other	(6)	(4)
Projected benefit obligation at end of year	1,036	1,029	1,184
Change in plan assets
Fair value of plan assets at beginning of year	1,080	1,160
Actual return/(loss) on plan assets	78	(64)
Employer contributions	5	4
Participant contributions	0	0
Benefits paid	(76)	(94)
Foreign currency exchange rate changes	(29)	78
Other	(6)	(4)
Fair value of plan assets at end of year	1,052	1,080	1,160
Overfunded status at end of year	16	51
Presented as follows:
Deferred amounts and other assets	116	141
Other current liabilities	(5)	(5)
Other long-term liabilities	(95)	(85)
Amount recognized in balance sheet	16	51
Accumulated benefit obligation	1,000	1,000
US | Supplemental Employee Retirement Plan
Presented as follows:
Deferred amounts and other assets	62	58
US | OPEB
Change in projected benefit obligation
Projected benefit obligation at beginning of year	136	173
Service cost	1	1	1
Interest cost	6	3	3
Participant contributions	5	6
Actuarial gain	4	(37)
Benefits paid	(20)	(21)
Foreign currency exchange rate changes	(3)	11
Projected benefit obligation at end of year	129	136	173
Change in plan assets
Fair value of plan assets at beginning of year	185	201
Actual return/(loss) on plan assets	14	(21)
Employer contributions	7	7
Participant contributions	5	6
Benefits paid	(20)	(21)
Foreign currency exchange rate changes	(4)	13
Fair value of plan assets at end of year	187	185	$ 201
Overfunded status at end of year	58	49
Presented as follows:
Deferred amounts and other assets	73	75
Other current liabilities	0	0
Other long-term liabilities	(15)	(26)
Amount recognized in balance sheet	$ 58	$ 49