CHANGES IN ACCOUNTING POLICIES - Narrative (Details) - CAD ($) $ in Millions	Jun. 30, 2020	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Additional deficit on Statements of Financial Position	$ (69,941)			$ (69,407)	$ (72,805)
Retained earnings/(deficit)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Additional deficit on Statements of Financial Position	$ 7,797	$ 7,808		6,314	$ 3,392	$ 3,640	$ 5,538
Adjustment | Retained earnings/(deficit)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Additional deficit on Statements of Financial Position		$ 0		$ 66
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