REVENUE - Non-member (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of revenue
Other	$ 12,128	$ 13,666	$ 39,540	$ 38,337
Revenues	399,053	398,157	1,053,558	1,044,178
Non-member electric sales and other operating revenue
Disaggregation of revenue
Revenues	40,467	44,670	111,383	101,262
Non-member electric sales
Disaggregation of revenue
Revenue from contract with customer	28,339	31,004	71,843	62,925
Non-member electric sales | Long-term contracts
Disaggregation of revenue
Revenue from contract with customer	12,647	11,985	35,241	34,457
Non-member electric sales | Short-term contracts
Disaggregation of revenue
Revenue from contract with customer	15,692	19,019	36,602	28,468
Other
Disaggregation of revenue
Other	$ 12,128	$ 13,666	$ 39,540	$ 38,337