SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Revenue (Details ) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 765,704	$ 697,577	$ 664,358
Regulatory balancing account revenue	25,205	96,730	50,199
Operating revenue	790,909	794,307	714,557
Non-regulated revenue	22,761	16,922	19,205
Residential
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	528,843	486,065	446,323
Business
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	144,565	125,819	129,223
Industrial
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	26,569	29,088	31,857
Public authorities
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	40,501	35,776	33,862
Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	25,226	20,829	23,093
Operating and maintenance revenue
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	16,276	11,481	12,655
Other non-regulated revenue
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	3,741	3,043	4,271
Non-regulated revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	20,017	14,524	16,926
Lease revenue	2,744	2,398	2,279
Non-regulated revenue	$ 22,761	$ 16,922	$ 19,205