PENSION AND OTHER POSTRETIREMENT BENEFITS - BENEFIT OBLIGATION, PLAN ASSETS AND FUNDED STATUS (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Canada
Change in plan assets
Employer contributions	$ 4	$ 13	$ 14
Canada | Pension
Change in projected benefit obligation
Accumulated postretirement benefit obligation at beginning of year	3,997	4,033
Service cost	149	149	156
Interest cost	139	130	116
Participant contributions	32	25
Actuarial (gain)/loss	423	(146)
Benefits paid	(187)	(184)
Plan settlements	(99)	0
Transfers out	(8)	(10)
Foreign currency exchange rate changes	0	0
Other	0	0
Accumulated postretirement benefit obligation at end of year	4,446	3,997	4,033
Change in plan assets
Fair value of plan assets at beginning of year	3,523	3,619
Actual return/(loss) on plan assets	448	(42)
Employer contributions	114	113
Participant contributions	32	25
Benefits paid	(187)	(184)
Plan settlements	(99)	0
Transfers out	(4)	(8)
Foreign currency exchange rate changes	0	0
Other	0	0
Fair value of plan assets at end of year	3,827	3,523	3,619
Underfunded status at end of year	(619)	(474)
Presented as follows:
Deferred amounts and other assets	35	29
Accounts payable and other	(9)	(9)
Other long-term liabilities	(645)	(494)
Amount recognized in balance sheet	(619)	(474)
Accumulated benefit obligation	4,000	3,700
Canada | Supplemental Employee Retirement Plan
Presented as follows:
Deferred amounts and other assets	10	7
Canada | OPEB
Change in projected benefit obligation
Accumulated postretirement benefit obligation at beginning of year	282	321
Service cost	5	8	7
Interest cost	10	10	10
Participant contributions	0	0
Actuarial (gain)/loss	15	(45)
Benefits paid	(6)	(11)
Plan settlements	0	0
Foreign currency exchange rate changes	0	0
Other	(13)	(1)
Accumulated postretirement benefit obligation at end of year	293	282	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	6	11
Participant contributions	0	0
Benefits paid	(6)	(11)
Foreign currency exchange rate changes	0	0
Other	0	0
Fair value of plan assets at end of year	0	0	0
Underfunded status at end of year	(293)	(282)
Presented as follows:
Deferred amounts and other assets	0	0
Accounts payable and other	(12)	(12)
Other long-term liabilities	(281)	(270)
Amount recognized in balance sheet	(293)	(282)
United States
Change in plan assets
Employer contributions	27	27	31
United States | Pension
Change in projected benefit obligation
Accumulated postretirement benefit obligation at beginning of year	1,214	1,279
Service cost	45	45	48
Interest cost	41	38	35
Participant contributions	0	0
Actuarial (gain)/loss	106	(103)
Benefits paid	(101)	(60)
Plan settlements	(1)	(65)
Transfers out	(6)	0
Foreign currency exchange rate changes	(63)	105
Other	(5)	(25)
Accumulated postretirement benefit obligation at end of year	1,230	1,214	1,279
Change in plan assets
Fair value of plan assets at beginning of year	1,045	1,097
Actual return/(loss) on plan assets	176	(48)
Employer contributions	46	40
Participant contributions	0	0
Benefits paid	(101)	(60)
Plan settlements	(1)	(65)
Transfers out	0	0
Foreign currency exchange rate changes	(56)	91
Other	(5)	(10)
Fair value of plan assets at end of year	1,104	1,045	1,097
Underfunded status at end of year	(126)	(169)
Presented as follows:
Deferred amounts and other assets	0	0
Accounts payable and other	(4)	(4)
Other long-term liabilities	(122)	(165)
Amount recognized in balance sheet	(126)	(169)
Accumulated benefit obligation	1,200	1,200
United States | Supplemental Employee Retirement Plan
Presented as follows:
Deferred amounts and other assets	51	39
United States | OPEB
Change in projected benefit obligation
Accumulated postretirement benefit obligation at beginning of year	305	337
Service cost	2	3	5
Interest cost	10	10	10
Participant contributions	5	6
Actuarial (gain)/loss	7	(25)
Benefits paid	(28)	(29)
Plan settlements	0	(8)
Foreign currency exchange rate changes	(15)	27
Other	2	(16)
Accumulated postretirement benefit obligation at end of year	288	305	337
Change in plan assets
Fair value of plan assets at beginning of year	181	213
Actual return/(loss) on plan assets	27	(13)
Employer contributions	10	8
Participant contributions	5	6
Benefits paid	(28)	(29)
Foreign currency exchange rate changes	(9)	16
Other	2	(20)
Fair value of plan assets at end of year	188	181	$ 213
Underfunded status at end of year	(100)	(124)
Presented as follows:
Deferred amounts and other assets	0	2
Accounts payable and other	(8)	(7)
Other long-term liabilities	(92)	(119)
Amount recognized in balance sheet	$ (100)	$ (124)