RISK MANAGEMENT AND FINANCIAL INSTRUMENTS - Other Financial Instruments (Details) $ in Millions	9 Months Ended
	Sep. 30, 2019 CAD ($)	Sep. 30, 2018 CAD ($)	Dec. 31, 2018 CAD ($)	Dec. 31, 2018 USD ($)
Fair Value of Other Financial Instruments
FMVA investments	$ 98,000,000		$ 102,000,000
Long-term debt, carrying value	65,700,000,000		63,900,000,000
Long-term debt	71,800,000,000		64,400,000,000
Net Investment Hedges
Fair Value of Other Financial Instruments
Unrealized foreign exchange gain (loss) on translation of United States dollar denominated debt	166,000,000	$ (209,000,000)
Unrealized gain (loss) on change in fair value of outstanding forward exchange forward contracts	1,000,000
Unrealized gain (loss) on change in fair value of outstanding forward exchange forward contracts		36,000,000
Realized gain (loss) associated with the settlement of foreign exchange forward contracts	0
Realized gain (loss) associated with the settlement of foreign exchange forward contracts		(46,000,000)
Realized gain (loss) associated with the settlement of United Stated dollar denominated debt that matured	0	$ (13,000,000)
Preferred share investment
Fair Value of Other Financial Instruments
Held to maturity investment at amortized cost	$ 580,000,000		478,000,000
Cumulative dividends based on average yield of Government of Canada bonds, maturity period of bonds	10 years
Cumulative dividends based on average yield of Government of Canada bonds, spread over reference rate (as a percent)	4.38%
Held to maturity investment, fair value	$ 580,000,000			$ 580
Fair Value
Fair Value of Other Financial Instruments
Restricted long-term investments held in trust	413,000,000		323,000,000
Noncurrent notes receivable	94,000,000		97,000,000
Carrying value
Fair Value of Other Financial Instruments
Noncurrent notes receivable	$ 94,000,000		$ 97,000,000