FAIR VALUE MEASUREMENTS AND DERIVATIVE INSTRUMENTS (Financial Impact of Derivative Energy Contracts) (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
Derivative Instruments, Gain (Loss) [Line Items]
Percent of gains shared with ratepayers									10.00%
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
Energy Derivative
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized Net Gain (Loss)									(9,000)	(18,000)	12,000
Operating Revenues									$ 5,000	$ 5,000	$ 4,000