DEBT - Narrative (Details)		3 Months Ended	5 Months Ended	6 Months Ended
	May 16, 2019 CAD ($)	Jun. 30, 2019 CAD ($)	Jun. 30, 2019 CAD ($)	Jun. 30, 2019 CAD ($)	May 16, 2019 JPY (¥)	Dec. 31, 2018 CAD ($)
Line of Credit Facility [Line Items]
Line of credit facility, increase (decrease), net			$ 444,000,000
Commitment fee percentage				0.10%
Long-term debt		$ 60,017,000,000	60,017,000,000	$ 60,017,000,000		$ 60,327,000,000
Long-term debt, fair value adjustment				898,000,000
Amortization of fair value adjustment		17,000,000		34,000,000
Extendible credit facility
Line of Credit Facility [Line Items]
Debt instrument, term	3 years
Amount of credit facility	$ 641,000,000				¥ 52,500,000,000
Uncommitted demand credit facilities
Line of Credit Facility [Line Items]
Amount of credit facility		887,000,000	887,000,000	887,000,000		807,000,000
Unutilized amount of facility		571,000,000	571,000,000	571,000,000		548,000,000
Commercial paper and credit facility draws
Line of Credit Facility [Line Items]
Long-term debt		12,181,000,000	12,181,000,000	12,181,000,000		7,967,000,000
Subordinated Debt | Fixed to Floating Subordinated Term Notes
Line of Credit Facility [Line Items]
Subordinated debt		$ 6,582,000,000	$ 6,582,000,000	$ 6,582,000,000		$ 7,317,000,000