NEW ACCOUNTING PRONOUNCEMENTS (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) customer	Jun. 30, 2016 USD ($)
NEW ACCOUNTING PRONOUNCEMENTS
Number of Members | customer			43
Member electric sales	$ 292,259	$ 275,517	$ 574,674	$ 547,286
Member sales as a percent of operating revenue			85.00%
Non-member electric sales	$ 25,313	$ 25,054	$ 60,471	$ 55,176
Member sales as a percent of operating revenue			9.00%