CONTRACT ASSETS AND CONTRACT LIABILITIES (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Contract assets
Contract assets	$ 0
Accounts receivable
Accounts receivable—Members	105,371	$ 107,572
Other accounts receivable	28,039	22,434
Contract liabilities
Contract liabilities (unearned revenue)	7,041	7,906
Other accounts receivable - trade
Accounts receivable
Other accounts receivable	25,355	13,004
Other accounts receivable - trade | Non-member electric sales
Accounts receivable
Other accounts receivable	4,727	6,998
Other accounts receivable - trade | Other
Accounts receivable
Other accounts receivable	20,628	6,006
Other accounts receivable, non-trade
Accounts receivable
Other accounts receivable	2,684	$ 9,430
Other operating revenue
Contract liabilities
Contract liabilities recognized	$ 700