CONTRACT ASSETS AND CONTRACT LIABILITIES (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Accounts receivable
Accounts receivable—Members	$ 109,565	$ 107,572
Other accounts receivable	$ 21,554	22,434
Initial royalty rate paid by Colowyo Mine for coal extracted under certain leases from September 1, 2015 through June 30, 2019	12.50%
Reduced royalty rate granted to Colowyo Mine by the Office of Natural Resources Revenue for coal extracted under certain leases from September 1, 2015 through June 30, 2019	8.00%
Contract liabilities
Contract liabilities (unearned revenue)	$ 7,257	7,906
Other accounts receivable - trade
Accounts receivable
Other accounts receivable	20,439	13,004
Other accounts receivable - trade | Non-member electric sales
Accounts receivable
Other accounts receivable	5,465	6,998
Other accounts receivable - trade | Other
Accounts receivable
Other accounts receivable	14,974	6,006
Other accounts receivable, non-trade
Accounts receivable
Other accounts receivable	1,115	$ 9,430
Other operating revenue
Contract liabilities
Contract liabilities recognized	$ 600