RISK MANAGEMENT AND FINANCIAL INSTRUMENTS - OTHER FINANCIAL INSTRUMENTS (Details) - CAD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Fair Value of Other Financial Instruments
FMVA investments	$ 100,000,000		$ 99,000,000
Long-term debt, carrying value	65,600,000,000		64,000,000,000
Long-term debt	68,000,000,000		67,400,000,000
Net Investment Hedges
Fair Value of Other Financial Instruments
Unrealized foreign exchange gain (loss) on translation of United States dollar denominated debt	194,000,000	$ 20,000,000
Unrealized gain (loss) on change in fair value of outstanding forward exchange forward contracts	15,000,000	9,000,000
Realized gain (loss) associated with the settlement of foreign exchange forward contracts	23,000,000	1,000,000
Realized gain (loss) associated with the settlement of United Stated dollar denominated debt that matured	11,000,000	20,000,000
Amount of ineffectiveness	0	$ 0
Preferred share investment
Fair Value of Other Financial Instruments
Held to maturity investment at amortized cost	$ 382,000,000		371,000,000
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)	4.50%
Held to maturity investment, fair value			580,000,000
Carrying value
Fair Value of Other Financial Instruments
Noncurrent notes receivable	$ 92,000,000		89,000,000
Fair Value
Fair Value of Other Financial Instruments
Restricted long-term investments held in trust	280,000,000		267,000,000
Noncurrent notes receivable	$ 92,000,000		$ 89,000,000