REVENUE (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers									$ 1,360,000	$ 1,272,000	$ 1,184,000
Alternative Revenues									28,000	24,000	20,000
Other									45,000	45,000	31,000
Total Operating Revenues	$ 344,000	$ 460,000	$ 354,000	$ 275,000	$ 304,000	$ 417,000	$ 352,000	$ 268,000	1,432,618	1,340,935	1,234,995
Retail
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers									1,022,000	1,017,000	969,000
Wholesale
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers									238,000	152,000	106,000
Other Services
Disaggregation of Revenue [Line Items]
Revenues from Contracts with Customers									$ 100,000	$ 103,000	$ 109,000
Lost Fixed Cost Recovery Mechanism
Disaggregation of Revenue [Line Items]
Cap on increase in lost fixed cost recovery rate (in percentage)									2.00%