PENSION AND OTHER POSTRETIREMENT BENEFITS - Benefit Obligation, Plan Assets and Funded Status (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Change in plan assets
Fair value of plan assets at beginning of year	$ 61
Fair value of plan assets at end of year	77	$ 61
Canada | Pension
Change in projected benefit obligation
Projected benefit obligation at beginning of year	4,092	3,630
Service cost	103	81	$ 131
Interest cost	186	184	127
Participant contributions	32	31
Actuarial (gain)/loss	0	359
Benefits paid	(210)	(193)
Transfer in	0	0
Foreign currency exchange rate changes	0	0
Other	0	0
Projected benefit obligation at end of year	4,203	4,092	3,630
Change in plan assets
Fair value of plan assets at beginning of year	4,528	4,234
Actual return on plan assets	627	427
Employer contributions	23	27
Participant contributions	32	31
Benefits paid	(210)	(193)
Transfer in	0	0
Foreign currency exchange rate changes	0	0
Other	0	2
Fair value of plan assets at end of year	5,000	4,528	4,234
Overfunded status at end of year	797	436
Presented as follows:
Deferred amounts and other assets	943	636
Other current liabilities	(10)	(8)
Other long-term liabilities	(136)	(192)
Amount recognized in balance sheet	797	436
Accumulated benefit obligation	3,900	3,800
Canada | Supplemental Employee Retirement Plan
Presented as follows:
Deferred amounts and other assets	18	14
Canada | OPEB
Change in projected benefit obligation
Projected benefit obligation at beginning of year	228	211
Service cost	3	3	4
Interest cost	10	11	7
Participant contributions	0	0
Actuarial (gain)/loss	(21)	13
Benefits paid	(10)	(10)
Transfer in	0	0
Foreign currency exchange rate changes	0	0
Projected benefit obligation at end of year	210	228	211
Change in plan assets
Fair value of plan assets at beginning of year	0	0
Actual return on plan assets	0	0
Employer contributions	10	10
Participant contributions	0	0
Benefits paid	(10)	(10)
Transfer in	0	0
Foreign currency exchange rate changes	0	0
Other	0	0
Fair value of plan assets at end of year	0	0	0
Overfunded status at end of year	(210)	(228)
Presented as follows:
Deferred amounts and other assets	0	0
Other current liabilities	(11)	(12)
Other long-term liabilities	(199)	(216)
Amount recognized in balance sheet	(210)	(228)
US
Change in plan assets
Employer contributions	35	33	30
US | Pension
Change in projected benefit obligation
Projected benefit obligation at beginning of year	1,036	1,029
Service cost	60	40	43
Interest cost	69	47	24
Participant contributions	0	0
Actuarial (gain)/loss	(98)	31
Benefits paid	(100)	(76)
Transfer in	569	0
Foreign currency exchange rate changes	119	(29)
Other	(7)	(6)
Projected benefit obligation at end of year	1,648	1,036	1,029
Change in plan assets
Fair value of plan assets at beginning of year	1,052	1,080
Actual return on plan assets	197	78
Employer contributions	5	5
Participant contributions	0	0
Benefits paid	(100)	(76)
Transfer in	900	0
Foreign currency exchange rate changes	146	(29)
Other	(6)	(6)
Fair value of plan assets at end of year	2,194	1,052	1,080
Overfunded status at end of year	546	16
Presented as follows:
Deferred amounts and other assets	653	116
Other current liabilities	(6)	(5)
Other long-term liabilities	(101)	(95)
Amount recognized in balance sheet	546	16
Accumulated benefit obligation	1,600	1,000
US | Supplemental Employee Retirement Plan
Presented as follows:
Deferred amounts and other assets	80	62
US | OPEB
Change in projected benefit obligation
Projected benefit obligation at beginning of year	129	136
Service cost	3	1	1
Interest cost	7	6	3
Participant contributions	5	5
Actuarial (gain)/loss	(9)	4
Benefits paid	(20)	(20)
Transfer in	46	0
Foreign currency exchange rate changes	16	(3)
Projected benefit obligation at end of year	177	129	136
Change in plan assets
Fair value of plan assets at beginning of year	187	185
Actual return on plan assets	22	14
Employer contributions	7	7
Participant contributions	5	5
Benefits paid	(20)	(20)
Transfer in	55	0
Foreign currency exchange rate changes	20	(4)
Other	2	0
Fair value of plan assets at end of year	278	187	$ 185
Overfunded status at end of year	101	58
Presented as follows:
Deferred amounts and other assets	113	73
Other current liabilities	0	0
Other long-term liabilities	(12)	(15)
Amount recognized in balance sheet	$ 101	$ 58