RISK MANAGEMENT AND FINANCIAL INSTRUMENTS - OTHER FINANCIAL INSTRUMENTS (Details) - CAD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Fair Value of Other Financial Instruments
Equity investments at carrying value	$ 99,000,000	$ 102,000,000
Restricted long-term investments	434,000,000	323,000,000
Long-term debt	70,500,000,000	64,400,000,000
Long-term loans receivable from affiliates	1,026,000,000	767,000,000
Net Investment Hedges
Fair Value of Other Financial Instruments
Unrealized foreign exchange gain (loss) on translation of United States dollar denominated debt	317,000,000	479,000,000
Unrealized gain (loss) on change in fair value of outstanding forward exchange forward contracts	2,000,000	30,000,000
Realized loss associated with the settlement of foreign exchange forward contracts	0	45,000,000
Realized gain (loss) associated with the settlement of United Stated dollar denominated debt that matured	0	14,000,000
Carrying value
Fair Value of Other Financial Instruments
Long-term debt	64,400,000,000	63,900,000,000
Preference shares
Fair Value of Other Financial Instruments
Held-to-maturity securities	$ 580,000,000	478,000,000
Cumulative dividends based on average yield of Government of Canada bonds, maturity period of bonds	10 years
Held to maturity preferred share investment	$ 580,000,000	$ 580,000,000
Noverco | Preference shares
Fair Value of Other Financial Instruments
Cumulative dividends based on average yield of Government of Canada bonds, spread over reference rate (as a percent)	4.38%