SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Revenue (Details ) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 697,577	$ 664,358	$ 674,736
Regulatory balancing account revenue	96,730	50,199	23,460
Total operating revenue	794,307	714,557	698,196
Total non-regulated revenue	16,922	19,205	18,272
Residential
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	486,065	446,323	450,062
Business
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	125,819	129,223	130,041
Industrial
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	29,088	31,857	34,236
Public authorities
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	35,776	33,862	34,511
Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	20,829	23,093	25,886
Operating and maintenance revenue
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	11,481	12,655	10,392
Other non-regulated revenue
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	3,043	4,271	5,413
Non-regulated revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	14,524	16,926	15,805
Lease revenue	2,398	2,279
Lease revenue			2,467
Total non-regulated revenue	$ 16,922	$ 19,205	$ 18,272