REGULATORY MATTERS (Cost Recovery Mechanisms and FERC Compliance) (Details) $ in Millions	1 Months Ended				3 Months Ended		9 Months Ended		12 Months Ended
	Jan. 31, 2018 USD ($)	Aug. 31, 2017 USD ($)	May 30, 2017 USD ($)	Jan. 31, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) $ / kWh	Sep. 30, 2017 USD ($)	Dec. 31, 2025	Dec. 31, 2017 USD ($)
Public Utilities, General Disclosures [Line Items]
Proceeds from settlement				$ 8
Reversal of previous provision			$ 5
Purchased Power and Fuel Adjustment Clause
Public Utilities, General Disclosures [Line Items]
Months approved rate in effect unless modified							12 months
Months of preceding period for true up component							12 months
Over-recovered costs for purchased power and fuel					$ 20		$ 20			$ 9
Renewable Energy Program
Public Utilities, General Disclosures [Line Items]
Approved spending budget	$ 54
Renewable Energy Program | Scenario, Forecast
Public Utilities, General Disclosures [Line Items]
Renewable energy target (percentage)									15.00%
Distributed generation requirement percent of target percentage (percentage)									30.00%
Demand Side Management
Public Utilities, General Disclosures [Line Items]
Recovery revenue							2	$ 2
Energy Efficiency Standards
Public Utilities, General Disclosures [Line Items]
Proposed spending budget		$ 23
Lost Fixed Cost Recovery Mechanism - Energy Efficiency Standards
Public Utilities, General Disclosures [Line Items]
Recovery revenue					$ 5	$ 6	$ 18	$ 17
Cap on increase in lost fixed cost recovery rate (percentage)							2.00%
May 2018 through March 2019 | Purchased Power and Fuel Adjustment Clause
Public Utilities, General Disclosures [Line Items]
Purchased power and fuel adjustment clause rate (dollars per kwh) | $ / kWh							0.0020
March 2017 through April 2018 | Purchased Power and Fuel Adjustment Clause
Public Utilities, General Disclosures [Line Items]
Purchased power and fuel adjustment clause rate (dollars per kwh) | $ / kWh							(0.0020)
May 2016 through February 2017 | Purchased Power and Fuel Adjustment Clause
Public Utilities, General Disclosures [Line Items]
Purchased power and fuel adjustment clause rate (dollars per kwh) | $ / kWh							0.0015