OTHER DEFERRED CHARGES (Details) $ in Thousands	1 Months Ended	12 Months Ended
	Oct. 31, 2017 USD ($) instrument	Dec. 31, 2016 instrument	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Oct. 12, 2017 instrument
Other deferred charges
Regulatory liabilities			$ 85,815	$ 81,824
Preliminary surveys and investigations			20,403	19,737
Advances to operating agents of jointly owned facilities			9,938	10,740
Interest rate swaps			8,357	4,311
Other			2,138	3,704
Total other deferred charges			40,836	38,492
Interest rate swaps, realized gain
Other deferred charges
Regulatory liabilities	$ 4,600		4,559	4,614
Amortization period	12 years
Interest rate swap - unrealized gain
Other deferred charges
Regulatory liabilities			$ 8,357	$ 4,311
Interest rate swaps
Other deferred charges
Number of instruments entered into | instrument		2
Number of instruments settled | instrument	1				1