NEW ACCOUNTING PRONOUNCEMENTS (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) customer	Sep. 30, 2016 USD ($)
NEW ACCOUNTING PRONOUNCEMENTS
Number of Members | customer			43
Member electric sales	$ 346,802	$ 326,251	$ 921,476	$ 873,537
Member sales as a percent of operating revenue			85.80%
Non-member electric sales	25,354	29,039	$ 85,825	84,215
Member sales as a percent of operating revenue			8.00%
Other Revenue, Net	$ 24,355	$ 23,062	$ 66,540	$ 64,684
Other operating revenue as a percent of operating revenue			6.20%