RISK MANAGEMENT AND FINANCIAL INSTRUMENTS - LEVEL 3 INPUTS (Details) CAD in Millions	12 Months Ended
	Dec. 31, 2017 CAD CAD / bbl CAD / Gallon-gal CAD / MWh CAD / MillionsofBTU-MMBTU	Dec. 31, 2016 CAD
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value	CAD (2,192)	CAD (4,150)
Fair Value
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value	(2,192)	(4,150)
Level 3 | Fair Value
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value	CAD (387)	CAD (295)
Market approach valuation technique | Level 3 | Commodity Contracts - Financial | Natural gas | Minimum
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price | CAD / MillionsofBTU-MMBTU	2.67
Market approach valuation technique | Level 3 | Commodity Contracts - Financial | Natural gas | Maximum
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price | CAD / MillionsofBTU-MMBTU	5.52
Market approach valuation technique | Level 3 | Commodity Contracts - Financial | Natural gas | Weighted Average
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price | CAD / MillionsofBTU-MMBTU	3.38
Market approach valuation technique | Level 3 | Commodity Contracts - Financial | Crude oil | Minimum
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price | CAD / bbl	43.76
Market approach valuation technique | Level 3 | Commodity Contracts - Financial | Crude oil | Maximum
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price | CAD / bbl	65.60
Market approach valuation technique | Level 3 | Commodity Contracts - Financial | Crude oil | Weighted Average
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price | CAD / bbl	51.03
Market approach valuation technique | Level 3 | Commodity Contracts - Financial | NGL | Minimum
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price | CAD / Gallon-gal	0.30
Market approach valuation technique | Level 3 | Commodity Contracts - Financial | NGL | Maximum
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price | CAD / Gallon-gal	1.83
Market approach valuation technique | Level 3 | Commodity Contracts - Financial | NGL | Weighted Average
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price | CAD / Gallon-gal	1.32
Market approach valuation technique | Level 3 | Commodity Contracts - Financial | Power | Minimum
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price | CAD / MWh	15.39
Market approach valuation technique | Level 3 | Commodity Contracts - Financial | Power | Maximum
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price | CAD / MWh	71.41
Market approach valuation technique | Level 3 | Commodity Contracts - Financial | Power | Weighted Average
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price | CAD / MWh	50.72
Market approach valuation technique | Level 3 | Commodity Contracts - Physical | Natural gas | Minimum
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price | CAD / MillionsofBTU-MMBTU	2.51
Market approach valuation technique | Level 3 | Commodity Contracts - Physical | Natural gas | Maximum
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price | CAD / MillionsofBTU-MMBTU	7.57
Market approach valuation technique | Level 3 | Commodity Contracts - Physical | Natural gas | Weighted Average
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price | CAD / MillionsofBTU-MMBTU	2.93
Market approach valuation technique | Level 3 | Commodity Contracts - Physical | Crude oil | Minimum
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price | CAD / bbl	34.38
Market approach valuation technique | Level 3 | Commodity Contracts - Physical | Crude oil | Maximum
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price | CAD / bbl	80.56
Market approach valuation technique | Level 3 | Commodity Contracts - Physical | Crude oil | Weighted Average
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price | CAD / bbl	69.01
Market approach valuation technique | Level 3 | Commodity Contracts - Physical | NGL | Minimum
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price | CAD / Gallon-gal	0.28
Market approach valuation technique | Level 3 | Commodity Contracts - Physical | NGL | Maximum
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price | CAD / Gallon-gal	1.94
Market approach valuation technique | Level 3 | Commodity Contracts - Physical | NGL | Weighted Average
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price | CAD / Gallon-gal	0.93
Market approach valuation technique | Level 3 | Fair Value | Commodity Contracts - Financial | Natural gas
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value	CAD (1)
Market approach valuation technique | Level 3 | Fair Value | Commodity Contracts - Financial | Crude oil
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value	(4)
Market approach valuation technique | Level 3 | Fair Value | Commodity Contracts - Financial | NGL
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value	(12)
Market approach valuation technique | Level 3 | Fair Value | Commodity Contracts - Financial | Power
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value	(110)
Market approach valuation technique | Level 3 | Fair Value | Commodity Contracts - Physical | Natural gas
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value	(114)
Market approach valuation technique | Level 3 | Fair Value | Commodity Contracts - Physical | Crude oil
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value	(148)
Market approach valuation technique | Level 3 | Fair Value | Commodity Contracts - Physical | NGL
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value	CAD 3
Option model valuation technique | Level 3 | Commodity Options | Crude oil | Minimum
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Option Volatility (as a percent)	15.00%
Option model valuation technique | Level 3 | Commodity Options | Crude oil | Maximum
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Option Volatility (as a percent)	24.00%
Option model valuation technique | Level 3 | Commodity Options | Crude oil | Weighted Average
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Option Volatility (as a percent)	22.00%
Option model valuation technique | Level 3 | Commodity Options | Power | Minimum
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Option Volatility (as a percent)	29.00%
Option model valuation technique | Level 3 | Commodity Options | Power | Maximum
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Option Volatility (as a percent)	55.00%
Option model valuation technique | Level 3 | Commodity Options | Power | Weighted Average
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Option Volatility (as a percent)	35.00%
Option model valuation technique | Level 3 | Fair Value | Commodity Options | Crude oil
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value	CAD (1)
Option model valuation technique | Level 3 | Fair Value | Commodity Options | Power
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value	CAD 0