INCOME TAXES - UNRECOGNIZED TAX BENEFITS (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
UNRECOGNIZED TAX BENEFITS
Unrecognized tax benefits at beginning of year	$ 129	$ 139
Gross increases for tax positions of current year	1	1
Gross decreases for tax positions of prior year	(1)	(1)
Change in translation of foreign currency	(3)	(4)
Lapses of statute of limitations	(5)	(6)
Unrecognized tax benefits at end of year	121	129
Interest and penalties expense (recovery) related to unrecognized tax benefits	3	3
Accrued interest and penalties related to unrecognized tax benefits	$ 17	$ 15