REGULATORY DECISION AND RELATED ACCOUNTING AND POLICY CHANGES - Additional Information (Details) $ in Thousands			1 Months Ended	9 Months Ended	12 Months Ended
	Aug. 28, 2020 utility	Jul. 09, 2012 utility	Feb. 28, 2014	Sep. 30, 2021 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2020 USD ($)
Public Utilities, General Disclosures [Line Items]
Number of utilities to adjust revenue requirement | utility		7
Requested collective rate of increase over revenue		28.00%
Approved collective revenue increase				$ 3,600
Adopted rate of return on common equity				9.50%
Number of regulated utilities that filed a rate case application | utility	12
Number of regulated utilities | utility	17
Percentage of hook up fee liability to be recorded on ICFA receipts			70.00%	70.00%
Percentage of deferred revenue to be recorded on ICFA receipts			30.00%
Remaining performance obligation				$ 18,900		$ 17,800
Portion of future ICFA funds					0.30
Percentage reversal of regulatory liability					30.00%
Reversal of regulatory liability					$ 3,400
Regulatory liability				7,557		$ 7,986
ICFA revenues
Public Utilities, General Disclosures [Line Items]
Regulatory liability				6,100	$ 7,900
Tax Cuts and Jobs Act
Public Utilities, General Disclosures [Line Items]
Regulatory liability				$ 1,500