REVENUE - Non-member (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of revenue
Other	$ 10,890	$ 14,256
Revenues	319,466	339,917
Non-member electric sales and other operating revenue
Disaggregation of revenue
Revenues	26,703	40,986
Non-member electric sales
Disaggregation of revenue
Revenue from contract with customer	15,813	26,730
Non-member electric sales | Long-term contracts
Disaggregation of revenue
Revenue from contract with customer	12,144	11,682
Non-member electric sales | Short-term contracts
Disaggregation of revenue
Revenue from contract with customer	3,669	15,048
Other
Disaggregation of revenue
Other	$ 10,890	$ 14,256