LONG-TERM INVESTMENTS - Narrative (Details) shares in Millions, CAD in Millions, $ in Billions	1 Months Ended	12 Months Ended
	Feb. 29, 2016 shares	Dec. 31, 2017 CAD	Dec. 31, 2016 CAD	Dec. 31, 2015 CAD	Dec. 31, 2017 USD ($)
Schedule of Equity Method Investments [Line Items]
Goodwill of investee		CAD 2,000	CAD 859
Amortizable assets of investee		643	687
Dividends received from equity investments		1,400	825	CAD 719
Asset impairment charge		CAD 4,565	CAD 1,620	CAD 536
Preference shares
Schedule of Equity Method Investments [Line Items]
Margin (as a percent)		4.38%			4.38%
Sabal Trail
Schedule of Equity Method Investments [Line Items]
Equity investment		CAD 2,300			$ 1.9
Noverco | Common shares
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)		38.90%	38.90%		38.90%
Reciprocal shareholding (as a percent)		1.90%	3.40%		1.90%
Shares purchased (in shares) | shares	1.2
Indirect pro-rata interest (as a percent)		0.70%	1.30%		0.70%
Reduction from reciprocal shareholding		CAD 102	CAD 102
Noverco | Preference shares
Schedule of Equity Method Investments [Line Items]
Maturity period		10 years
Margin (as a percent)		4.38%			4.38%
Aux Sable
Schedule of Equity Method Investments [Line Items]
Asset impairment charge			CAD 37
Liquids Pipelines | Eddystone Rail Company, LLC
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)		100.00%	75.00%		100.00%
Investment impairment			CAD 184
Equity investment		CAD 0	19
Gas Transmission and Midstream | Aux Sable
Schedule of Equity Method Investments [Line Items]
Equity investment		CAD 300	324
Gas Transmission and Midstream | Sabal Trail
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)		50.00%			50.00%
Equity investment		CAD 2,355	CAD 0