DEBT - Credit Facilities (Narrative) (Details)				12 Months Ended
	Jul. 18, 2019 CAD ($)	May 16, 2019 CAD ($)	Feb. 08, 2019 CAD ($)	Dec. 31, 2019 CAD ($)	May 16, 2019 JPY (¥)	Dec. 31, 2018 CAD ($)
Line of Credit Facility [Line Items]
Line of credit facility, increase (decrease)			$ 444,000,000
Weighted average standby fee (as a percent)				0.10%
Long-term debt				$ 59,661,000,000		$ 60,327,000,000
Commercial paper and credit facility draws
Line of Credit Facility [Line Items]
Long-term debt				8,974,000,000		7,967,000,000
Extendible credit facility
Line of Credit Facility [Line Items]
Term of credit facility		3 years
Credit facility capacity		$ 641,000,000			¥ 52,500,000,000
Non-revolving, bilateral credit facility
Line of Credit Facility [Line Items]
Term of credit facility	5 years
Credit facility capacity	$ 500,000,000
Uncommitted credit facilities
Line of Credit Facility [Line Items]
Credit facility capacity				916,000,000		807,000,000
Unutilized credit facility				$ 476,000,000		$ 548,000,000