COMMITMENTS AND CONTINGENCIES - Supplemental Balance Sheet Information (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Operating leases
Other assets: Other	$ 15,589	$ 14,402
Other accrued liabilities	1,757	1,452
Other long-term liabilities	13,868	12,928
Total	15,625	14,380
Finance leases
Depreciable plant and equipment	18,207	18,207
Accumulated depreciation and amortization	(10,813)	(9,644)
Net utility plant	7,394	8,563
Current maturities of long-term debt, net	721	680
Long-term debt, net	4,483	5,205
Total finance lease liabilities	$ 5,204	$ 5,885
Weighted average remaining lease term
Operating leases	135 months	152 months
Finance leases	64 months	76 months
Weighted average discount rate
Operating leases	3.50%	3.70%
Finance leases	5.50%	5.50%
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	us-gaap:OtherAssets
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	ug-gaap:AccruedLiabilitiesAndOtherLiabilities	ug-gaap:AccruedLiabilitiesAndOtherLiabilities
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	us-gaap:LiabilitiesNoncurrentAbstract
Operating Lease, Liability, Statement of Financial Position [Extensible List]	us-gaap:LiabilitiesCurrent	us-gaap:LiabilitiesCurrent