QUARTERLY FINANCIAL DATA (UNAUDITED) (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
QUARTERLY FINANCIAL DATA (UNAUDITED)
Operating revenues	$ 331,914	$ 399,053	$ 314,588	$ 339,917	$ 276,659	$ 398,157	$ 327,513	$ 318,508	$ 1,385,472	$ 1,320,837	$ 1,388,593
Operating margin	13,616	77,347	34,945	40,517	(2,015)	81,393	41,716	40,299	166,425	161,393	183,697
Net margins attributable to the Association	(15,440)	$ 55,145	$ (1,385)	$ 6,989	$ (16,136)	$ 46,398	$ 4,378	$ 8,094	45,309	42,734	61,656
Deferred non-member electric sales revenue recognized
QUARTERLY FINANCIAL DATA (UNAUDITED)
Revenue from contract with customer	$ 6,200								$ 6,153
Deferred non-member electric sales revenue recognized | Calculated under Revenue Guidance in Effect before Topic 606
QUARTERLY FINANCIAL DATA (UNAUDITED)
Revenue from contract with customer											$ 5,473
Deferred non-member electric sales
QUARTERLY FINANCIAL DATA (UNAUDITED)
Revenue from contract with customer										$ (51,700)