LONG-TERM INVESTMENTS - Narrative (Details) - CAD ($) shares in Millions, $ in Millions	1 Months Ended		12 Months Ended
	Jan. 31, 2019	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]
Goodwill of investee		$ 2,200	$ 2,100	$ 2,200
Amortizable assets of investee		$ 706	681	706
Dividends received from equity investments			$ 2,200	$ 2,800	$ 1,400
Common Shares | Noverco
Schedule of Equity Method Investments [Line Items]
Reciprocal shareholding (as a percent)		1.40%	0.50%	1.40%
Equity Method Investment, number of shares sold (in shares)	11.6	4.4
Indirect pro-rata interest (as a percent)		0.50%	0.20%	0.50%
Reduction from reciprocal shareholding			$ 51	$ 88
Preference shares | Noverco
Schedule of Equity Method Investments [Line Items]
Maturity period			10 years
Margin (as a percent)			4.38%
Gas Distribution and Storage | Common Shares | Noverco
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)		38.90%	38.90%	38.90%