PENSION AND OTHER POSTRETIREMENT BENEFITS - Benefit Obligation, Plan Assets and Funded Status (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Presented as follows:
Accumulated benefit obligation		$ 1,100
Canada | Pension
Change in projected benefit obligation
Accumulated postretirement benefit obligation at beginning of year	$ 4,600	4,855
Service cost	131	139	$ 148
Interest cost	127	101	128
Participant contributions	29	28
Actuarial gain	(1,069)	(329)
Benefits paid	(187)	(194)
Foreign currency exchange rate changes	0	0
Other	(1)	0
Accumulated postretirement benefit obligation at end of year	3,630	4,600	4,855
Change in plan assets
Fair value of plan assets at beginning of year	4,536	4,077
Actual return/(loss) on plan assets	(235)	505
Employer contributions	91	120
Participant contributions	29	28
Benefits paid	(187)	(194)
Foreign currency exchange rate changes	0	0
Other	0	0
Fair value of plan assets at end of year	4,234	4,536	4,077
Overfunded/(underfunded) status at end of year	604	(64)
Presented as follows:
Deferred amounts and other assets	764	250
Accounts payable and other	(9)	(9)
Other long-term liabilities	(151)	(305)
Amount recognized in balance sheet	604	(64)
Accumulated benefit obligation	3,400	4,300
Canada | Supplemental Employee Retirement Plan
Presented as follows:
Deferred amounts and other assets	10	13
Canada | OPEB
Change in projected benefit obligation
Accumulated postretirement benefit obligation at beginning of year	274	321
Service cost	4	6	5
Interest cost	7	7	8
Participant contributions	0	0
Actuarial gain	(66)	(51)
Benefits paid	(8)	(9)
Foreign currency exchange rate changes	0	0
Other	0	0
Accumulated postretirement benefit obligation at end of year	211	274	321
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	8	9
Participant contributions	0	0
Benefits paid	(8)	(9)
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	(211)	(274)
Presented as follows:
Deferred amounts and other assets	0	0
Accounts payable and other	(12)	(12)
Other long-term liabilities	(199)	(262)
Amount recognized in balance sheet	(211)	(274)
United States
Change in plan assets
Employer contributions	30	27	27
United States | Pension
Change in projected benefit obligation
Accumulated postretirement benefit obligation at beginning of year	1,184	1,243
Service cost	43	44	44
Interest cost	24	17	31
Participant contributions	0	0
Actuarial gain	(201)	(21)
Benefits paid	(94)	(84)
Foreign currency exchange rate changes	77	(11)
Other	(4)	(4)
Accumulated postretirement benefit obligation at end of year	1,029	1,184	1,243
Change in plan assets
Fair value of plan assets at beginning of year	1,160	1,062
Actual return/(loss) on plan assets	(64)	151
Employer contributions	4	43
Participant contributions	0	0
Benefits paid	(94)	(84)
Foreign currency exchange rate changes	78	(8)
Other	(4)	(4)
Fair value of plan assets at end of year	1,080	1,160	1,062
Overfunded/(underfunded) status at end of year	51	(24)
Presented as follows:
Deferred amounts and other assets	141	98
Accounts payable and other	(5)	(4)
Other long-term liabilities	(85)	(118)
Amount recognized in balance sheet	51	(24)
Accumulated benefit obligation	1,000
United States | Supplemental Employee Retirement Plan
Presented as follows:
Deferred amounts and other assets	58	84
United States | OPEB
Change in projected benefit obligation
Accumulated postretirement benefit obligation at beginning of year	173	254
Service cost	1	1	2
Interest cost	3	3	7
Participant contributions	6	8
Actuarial gain	(37)	(69)
Benefits paid	(21)	(22)
Foreign currency exchange rate changes	11	(3)
Other	0	1
Accumulated postretirement benefit obligation at end of year	136	173	254
Change in plan assets
Fair value of plan assets at beginning of year	201	188
Actual return/(loss) on plan assets	(21)	22
Employer contributions	7	6
Participant contributions	6	8
Benefits paid	(21)	(22)
Foreign currency exchange rate changes	13	(3)
Other	0	2
Fair value of plan assets at end of year	185	201	$ 188
Overfunded/(underfunded) status at end of year	49	28
Presented as follows:
Deferred amounts and other assets	75	71
Accounts payable and other	0	0
Other long-term liabilities	(26)	(43)
Amount recognized in balance sheet	$ 49	$ 28