FAIR VALUE MEASUREMENTS AND DERIVATIVE INSTRUMENTS (Financial Impact of Derivative Energy Contracts) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Percent of gains shared with ratepayers									10.00%
Operating Revenues	$ 318,000	$ 441,000	$ 326,000	$ 333,000	$ 344,000	$ 460,000	$ 354,000	$ 275,000	$ 1,418,338	$ 1,432,618	$ 1,340,935
Energy Derivative | Not Designated
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized Net Loss									(45,000)	(9,000)	(18,000)
Operating Revenues									$ 6,000	$ 5,000	$ 5,000