CONTRACT ASSETS AND CONTRACT LIABILITIES (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Contract assets
Contract assets	$ 0
Accounts receivable
Accounts receivable—Members	94,151	$ 102,035
Other accounts receivable	24,592	16,034
Contract liabilities
Contract liabilities (unearned revenue)	7,366	7,567
Other accounts receivable - trade
Accounts receivable
Other accounts receivable	14,485	13,573
Other accounts receivable - trade | Non-member electric sales
Accounts receivable
Other accounts receivable	6,061	5,493
Other accounts receivable - trade | Coal Sales
Accounts receivable
Other accounts receivable		1,446
Other accounts receivable - trade | Other
Accounts receivable
Other accounts receivable	8,424	6,634
Other accounts receivable, non-trade
Accounts receivable
Other accounts receivable	10,107	$ 2,461
Other operating revenue
Contract liabilities
Contract liabilities recognized	$ 200