DEBT - Credit Facilities (Narrative) (Details)					12 Months Ended
	Feb. 10, 2021 USD ($)	Jul. 24, 2020 CAD ($)	Feb. 25, 2020 USD ($) creditFacility	Feb. 24, 2020 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2020 CAD ($)	Apr. 09, 2020 CAD ($)	Mar. 31, 2020 CAD ($)	Dec. 31, 2019 CAD ($)
Line of Credit Facility [Line Items]
Long-term debt						$ 62,819,000,000			$ 59,661,000,000
Extinguishment of debt					$ 2,100,000,000	1,700,000,000
Commercial paper and credit facility draws
Line of Credit Facility [Line Items]
Long-term debt						9,900,000,000			9,000,000,000.0
Non-revolving, bilateral credit facility
Line of Credit Facility [Line Items]
Term of credit facility			1 year
Credit facility capacity			$ 500,000,000
Number of credit facilities | creditFacility			2
Uncommitted credit facilities
Line of Credit Facility [Line Items]
Credit facility capacity						849,000,000			916,000,000
Unutilized credit facility						$ 533,000,000			$ 476,000,000
Weighted average standby fee (as a percent)					30.00%	30.00%
Non-revolving Credit Facility
Line of Credit Facility [Line Items]
Term of credit facility				2 years
Credit facility capacity				$ 1,000,000,000.0
Revolving Credit Facility
Line of Credit Facility [Line Items]
Credit facility capacity							$ 3,000,000,000.0	$ 1,700,000,000
Revolving Credit Facility | Extendible credit facility
Line of Credit Facility [Line Items]
Term of credit facility		364 days
Borrowing capacity		$ 10,000,000,000.0
Maturity period		1 year
Revolving Credit Facility | Subsequent Event | Sustainability-linked Credit Facility
Line of Credit Facility [Line Items]
Term of credit facility	3 years
Credit facility capacity	$ 1,000,000,000.0
Revolving Credit Facility | Subsequent Event | Syndicated Credit Facility
Line of Credit Facility [Line Items]
Term of credit facility	1 year
Extinguishment of debt	$ 3,000,000,000.0