SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Other Deferred Charges (Details) $ in Thousands	1 Months Ended
	Oct. 31, 2017 instrument	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Other deferred charges
Regulatory liabilities		$ 81,824	$ 95,512
Preliminary surveys and investigations		19,737	111,592
Advances to operating agents of jointly owned facilities		10,740	11,871
Interest rate swaps		4,311	12,140
Other		3,704	3,775
Total other deferred charges		38,492	139,378
Interest rate swaps
Other deferred charges
Number of instruments settled | instrument	1
Interest rate swaps, realized gain
Other deferred charges
Regulatory liabilities		4,614
Amortization period	12 years
Interest rate swap - unrealized gain
Other deferred charges
Regulatory liabilities		4,311	12,140
Holcomb Expansion
Other deferred charges
Preliminary surveys and investigations		$ 0	$ 91,300