RISK MANAGEMENT AND FINANCIAL INSTRUMENTS - NOTIONAL PRINCIPAL OR QUANTITY INFORMATION (Details) € in Millions, ¥ in Millions, £ in Millions, MWh in Millions, MMBbls in Millions, CAD in Millions, Bcf in Millions, $ in Millions	Dec. 31, 2017 CAD MWh Bcf MMBbls	Dec. 31, 2017 USD ($) MWh Bcf MMBbls	Dec. 31, 2017 JPY (¥) MWh Bcf MMBbls	Dec. 31, 2017 GBP (£) MWh Bcf MMBbls	Dec. 31, 2017 EUR (€) MWh Bcf MMBbls	Dec. 31, 2016 CAD MWh Bcf MMBbls	Dec. 31, 2016 USD ($) MWh Bcf MMBbls	Dec. 31, 2016 JPY (¥) MWh Bcf MMBbls	Dec. 31, 2016 GBP (£) MWh Bcf MMBbls	Dec. 31, 2016 EUR (€) MWh Bcf MMBbls
Foreign exchange contracts - United States dollar or GBP or Japanese yen forwards - purchase
RISK MANAGEMENT AND FINANCIAL INSTRUMENTS
2018 Notional Amount		$ 755	¥ 0	£ 18	€ 280
2019 Notional Amount		2	32,662	0	375
2020 Notional Amount		2	0	0	0
2021 Notional Amount		0	0	0	0
2022 Notional amount		0	20,000	0	0
Thereafter		0	¥ 0	0	0
Total Notional Amount Outstanding							$ 997	¥ 32,662	£ 97	€ 0
Foreign exchange contracts - United States dollar or GBP forwards - sell
RISK MANAGEMENT AND FINANCIAL INSTRUMENTS
2018 Notional Amount		4,478		0	0
2019 Notional Amount		3,246		89	0
2020 Notional Amount		3,258		25	35
2021 Notional Amount		1,689		27	169
2022 Notional amount		1,676		28	169
Thereafter		$ 1,820		£ 149	€ 889
Total Notional Amount Outstanding							$ 13,591		£ 285	€ 0
Interest rate contracts - short-term borrowings
RISK MANAGEMENT AND FINANCIAL INSTRUMENTS
2018 Notional Amount	CAD 4,950
2019 Notional Amount	1,585
2020 Notional Amount	215
2021 Notional Amount	95
2022 Notional amount	91
Thereafter	202
Total Notional Amount Outstanding						CAD 14,008
Interest rate contracts - long-term debt
RISK MANAGEMENT AND FINANCIAL INSTRUMENTS
2018 Notional Amount	1,522
2019 Notional Amount	1,018
2020 Notional Amount	822
2021 Notional Amount	433
2022 Notional amount	349
Thereafter	52
Total Notional Amount Outstanding						0
Interest rate contract - long term debt - Pay Fixed Rate
RISK MANAGEMENT AND FINANCIAL INSTRUMENTS
2018 Notional Amount	4,007
2019 Notional Amount	957
2020 Notional Amount	438
2021 Notional Amount	0
2022 Notional amount	0
Thereafter	0
Total Notional Amount Outstanding						7,509
Equity contracts
RISK MANAGEMENT AND FINANCIAL INSTRUMENTS
2018 Notional Amount	45
2019 Notional Amount	37
2020 Notional Amount	8
2021 Notional Amount	0
2022 Notional amount	0
Thereafter	CAD 0
Total Notional Amount Outstanding						CAD 88
Commodity contracts | Natural gas
RISK MANAGEMENT AND FINANCIAL INSTRUMENTS
2018 Nonmonetary Notional Amount | Bcf	(59)	(59)	(59)	(59)	(59)
2019 Nonmonetary Notional Amount | Bcf	(69)	(69)	(69)	(69)	(69)
2020 Nonmonetary Notional Amount | Bcf	(20)	(20)	(20)	(20)	(20)
2021 Nonmonetary Notional amount | Bcf	(10)	(10)	(10)	(10)	(10)
2022 Nonmonetary Notional Amount | Bcf	(1)	(1)	(1)	(1)	(1)
Thereafter | Bcf	0	0	0	0	0
Derivative Nonmonetary Notional Amount Outstanding | Bcf						(161)	(161)	(161)	(161)	(161)
Commodity contracts | Crude oil
RISK MANAGEMENT AND FINANCIAL INSTRUMENTS
2018 Nonmonetary Notional Amount | MMBbls	(3)	(3)	(3)	(3)	(3)
2019 Nonmonetary Notional Amount | MMBbls	0	0	0	0	0
2020 Nonmonetary Notional Amount | MMBbls	0	0	0	0	0
2021 Nonmonetary Notional amount | MMBbls	0	0	0	0	0
2022 Nonmonetary Notional Amount | MMBbls	0	0	0	0	0
Thereafter | MMBbls	0	0	0	0	0
Derivative Nonmonetary Notional Amount Outstanding | MMBbls						(20)	(20)	(20)	(20)	(20)
Commodity contracts | NGL
RISK MANAGEMENT AND FINANCIAL INSTRUMENTS
2018 Nonmonetary Notional Amount | MMBbls	(12)	(12)	(12)	(12)	(12)
2019 Nonmonetary Notional Amount | MMBbls	0	0	0	0	0
2020 Nonmonetary Notional Amount | MMBbls	0	0	0	0	0
2021 Nonmonetary Notional amount | MMBbls	0	0	0	0	0
2022 Nonmonetary Notional Amount | MMBbls	0	0	0	0	0
Thereafter | MMBbls	0	0	0	0	0
Derivative Nonmonetary Notional Amount Outstanding | MMBbls						(14)	(14)	(14)	(14)	(14)
Commodity contracts | Power
RISK MANAGEMENT AND FINANCIAL INSTRUMENTS
2016/2017 | MWh	42	42	42	42	42
2017/2018 | MWh	51	51	51	51	51
2018/2019 | MWh	55	55	55	55	55
2019/2020 | MWh	(3)	(3)	(3)	(3)	(3)
2020/2021 | MWh	(43)	(43)	(43)	(43)	(43)
Thereafter | MWh	(43)	(43)	(43)	(43)	(43)
Derivative Nonmonetary Rate Notional Amount Outstanding | MWh						(4)	(4)	(4)	(4)	(4)