RISK MANAGEMENT AND FINANCIAL INSTRUMENTS - OTHER FINANCIAL INSTRUMENTS (Details) - CAD	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair Value of Other Financial Instruments
Equity investments at carrying value	CAD 99,000,000	CAD 110,000,000
Restricted investments, at fair value	267,000,000	90,000,000
Long-term debt	67,400,000,000	43,900,000,000
Notes receivable, noncurrent	89,000,000	0
Notes receivable, fair value	89,000,000	0
Net Investment Hedges
Fair Value of Other Financial Instruments
Unrealized foreign exchange gain (loss) on translation of United States dollar denominated debt	367,000,000	121,000,000
Unrealized gain (loss) on change in fair value of outstanding forward exchange forward contracts	286,000,000	21,000,000
Realized gain associated with the settlement of foreign exchange forward contracts	(198,000,000)	3,000,000
Realized gain (loss) associated with the settlement of United Stated dollar denominated debt that matured	23,000,000	26,000,000
Amount of ineffectiveness	CAD 0
Preference shares
Fair Value of Other Financial Instruments
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%
Carrying value
Fair Value of Other Financial Instruments
Long-term debt	CAD 64,000,000,000	40,800,000,000
Carrying value | Preference shares
Fair Value of Other Financial Instruments
Held to maturity preferred share investment	371,000,000	CAD 355,000,000
Fair Value | Preference shares
Fair Value of Other Financial Instruments
Held to maturity preferred share investment	CAD 580,000,000