CHANGES IN ACCOUNTING POLICIES - Narrative (Details) - CAD ($) $ in Millions		12 Months Ended
	Jan. 01, 2020	Dec. 31, 2019	Sep. 30, 2020	Jun. 30, 2020	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Additional deficit on Statements of Financial Position		$ (69,407)	$ (68,435)		$ (72,676)
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member	us-gaap:AccountingStandardsUpdate201613Member
Retained earnings/(deficit)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Additional deficit on Statements of Financial Position		$ 6,314	$ 8,442	$ 7,797	$ 3,932	$ 3,392	$ 5,538
Adjustment | Retained earnings/(deficit)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Additional deficit on Statements of Financial Position		$ 66		$ 0
Adjustment | Retained earnings/(deficit) | ASU 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Additional deficit on Statements of Financial Position	$ 66