COMPONENTS OF ACCUMULATED OTHER COMPREHENSIVE INCOME/(LOSS) (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Components of accumulated other comprehensive loss
Balance at beginning of year	$ 69,407	$ 73,435
Other comprehensive income/(loss) retained in AOCI	(1,522)	(3,296)	$ 3,479
Changes in AOCI
Amortization of pension and OPEB actuarial loss and prior service costs	17	17	16
Other comprehensive (income)/loss reclassified to earnings	(1,249)	(3,121)	3,680
Income tax on amounts retained in AOCI	182	164	148
Income tax on amounts reclassified to earnings	(62)	(35)	(41)
Tax impact	120	129	107
Sponsored vehicle buy-in and Other		48	(142)
Balance at end of year	64,363	69,407	73,435
Interest rate contracts
Changes in AOCI
Amortization of pension and OPEB actuarial loss and prior service costs	253	157	157
Commodity contracts
Changes in AOCI
Amortization of pension and OPEB actuarial loss and prior service costs		(1)	(1)
Forward exchange contracts
Changes in AOCI
Amortization of pension and OPEB actuarial loss and prior service costs	5	5	7
Other contracts
Changes in AOCI
Amortization of pension and OPEB actuarial loss and prior service costs	(2)	(3)	22
Cash Flow Hedges
Components of accumulated other comprehensive loss
Balance at beginning of year	(1,073)	(770)	(644)
Other comprehensive income/(loss) retained in AOCI	(591)	(599)	(244)
Changes in AOCI
Other comprehensive (income)/loss reclassified to earnings	(335)	(441)	(59)
Income tax on amounts retained in AOCI	140	169	57
Income tax on amounts reclassified to earnings	(58)	(31)	(37)
Tax impact	82	138	20
Sponsored vehicle buy-in and Other		0	(87)
Balance at end of year	(1,326)	(1,073)	(770)
Cash Flow Hedges | Interest rate contracts
Changes in AOCI
Amortization of pension and OPEB actuarial loss and prior service costs	253	157	157
Cash Flow Hedges | Commodity contracts
Changes in AOCI
Amortization of pension and OPEB actuarial loss and prior service costs		(1)	(1)
Cash Flow Hedges | Forward exchange contracts
Changes in AOCI
Amortization of pension and OPEB actuarial loss and prior service costs	5	5	7
Cash Flow Hedges | Other contracts
Changes in AOCI
Amortization of pension and OPEB actuarial loss and prior service costs	(2)	(3)	22
Excluded Components of Fair Value Hedges
Components of accumulated other comprehensive loss
Balance at beginning of year	0
Other comprehensive income/(loss) retained in AOCI	5
Changes in AOCI
Other comprehensive (income)/loss reclassified to earnings	5
Tax impact	0
Balance at end of year	5	0
Net Investment Hedges
Components of accumulated other comprehensive loss
Balance at beginning of year	(317)	(598)	(139)
Other comprehensive income/(loss) retained in AOCI	115	320	(509)
Changes in AOCI
Other comprehensive (income)/loss reclassified to earnings	115	320	(509)
Income tax on amounts retained in AOCI	(13)	(39)	50
Tax impact	(13)	(39)	50
Balance at end of year	(215)	(317)	(598)
Cumulative Translation Adjustment
Components of accumulated other comprehensive loss
Balance at beginning of year	1,396	4,323	77
Other comprehensive income/(loss) retained in AOCI	(828)	(2,927)	4,301
Changes in AOCI
Other comprehensive (income)/loss reclassified to earnings	(828)	(2,927)	4,301
Sponsored vehicle buy-in and Other		0	(55)
Balance at end of year	568	1,396	4,323
Equity Investees
Components of accumulated other comprehensive loss
Balance at beginning of year	67	34	10
Other comprehensive income/(loss) retained in AOCI	(2)	34	16
Changes in AOCI
Other comprehensive (income)/loss reclassified to earnings	(2)	34	16
Income tax on amounts retained in AOCI	1	6	8
Tax impact	1	6	8
Sponsored vehicle buy-in and Other		(7)	0
Balance at end of year	66	67	34
Pension and OPEB Adjustment
Components of accumulated other comprehensive loss
Balance at beginning of year	(345)	(317)	(277)
Other comprehensive income/(loss) retained in AOCI	(221)	(124)	(85)
Changes in AOCI
Amortization of pension and OPEB actuarial loss and prior service costs	17	17	16
Other comprehensive (income)/loss reclassified to earnings	(204)	(107)	(69)
Income tax on amounts retained in AOCI	54	28	33
Income tax on amounts reclassified to earnings	(4)	(4)	(4)
Tax impact	50	24	29
Sponsored vehicle buy-in and Other		55	0
Balance at end of year	(499)	(345)	(317)
Total
Components of accumulated other comprehensive loss
Balance at beginning of year	(272)	2,672	(973)
Changes in AOCI
Balance at end of year	$ (1,401)	$ (272)	$ 2,672