SIGNIFICANT ACCOUNTING POLICIES AND OTHER FINANCIAL DATA - OTHER INTANGIBLE ASSETS (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, Gross	$ 698	$ 632
Accumulated amortization	(102)	(84)
Finite-lived intangible assets, Net	596	548
Intangible assets amortization expense	18	11	$ 10
Future intangible asset amortization expense per year	21
Development rights [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, Gross	322	322
Accumulated amortization	$ (60)	(53)
Amortization period	50 years
Renewable energy and consumption permit [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, Gross	$ 154	154
Accumulated amortization	$ (8)	0
Amortization period	19 years
Storage rights [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, Gross	$ 138	138
Accumulated amortization	$ (28)	(25)
Amortization period	46 years
O&M agreement [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, Gross	$ 66	0
Amortization period	23 years
Other [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, Gross	$ 18	18
Accumulated amortization	$ (6)	$ (6)
Other [Member] | Minimum [Member]
Finite-Lived Intangible Assets [Line Items]
Amortization period	10 years