REGULATORY ASSETS AND LIABILITIES - Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Regulatory Assets and Liabilities
Regulatory Assets	$ 285,692	$ 325,376
Regulatory Liabilities	180,428	180,718
Future tax benefits due to customers
Regulatory Assets and Liabilities
Regulatory Liabilities	135,027	151,011
Postretirement Health Coverage
Regulatory Assets and Liabilities
Regulatory Liabilities	27,294	18,472
HCBA
Regulatory Assets and Liabilities
Regulatory Liabilities	9,687	5,320
Conservation program
Regulatory Assets and Liabilities
Regulatory Liabilities	7,206	3,837
Net WRAM and MCBA long-term payable
Regulatory Assets and Liabilities
Regulatory Liabilities	143	479
Other Regulatory Assets (Liabilities)
Regulatory Assets and Liabilities
Regulatory Liabilities	1,071	1,599
Pension Costs
Regulatory Assets and Liabilities
Regulatory Assets	17,607	59,588
Property-related temporary differences (tax benefits flowed through to customers)
Regulatory Assets and Liabilities
Regulatory Assets	130,565	120,365
Other accrued benefits
Regulatory Assets and Liabilities
Regulatory Assets	23,280	21,692
Regulatory Clause Revenues, under-recovered
Regulatory Assets and Liabilities
Regulatory Assets	29,789	33,136
Asset retirement obligations, net
Regulatory Assets and Liabilities
Regulatory Assets	22,935	21,110
IRMA long-term accounts receivable
Regulatory Assets and Liabilities
Regulatory Assets	$ 9,032	14,705
Tank coating
Regulatory Assets and Liabilities
Recovery Period	10 years
Regulatory Assets	$ 13,680	14,018
Recoverable property losses
Regulatory Assets and Liabilities
Recovery Period	8 years
Regulatory Assets	$ 3,843	4,531
PCBA
Regulatory Assets and Liabilities
Recovery Period	1 year
Regulatory Assets	$ 21,500	19,647
Other components of net periodic benefit cost
Regulatory Assets and Liabilities
Regulatory Assets	$ 3,342	6,736
General district balancing account receivable
Regulatory Assets and Liabilities
Recovery Period	1 year
Regulatory Assets	$ 568	1,830
Customer assistance program (CAP) and Rate support fund (RSF) accounts receivable
Regulatory Assets and Liabilities
Recovery Period	1 year
Regulatory Assets	$ 5,991	5,310
Other regulatory assets
Regulatory Assets and Liabilities
Regulatory Assets	$ 3,560	$ 2,708
Minimum | Net WRAM and MCBA long-term accounts receivable
Regulatory Assets and Liabilities
Recovery Period	1 year
Minimum | IRMA long-term accounts receivable
Regulatory Assets and Liabilities
Recovery Period	1 year
Maximum | Net WRAM and MCBA long-term accounts receivable
Regulatory Assets and Liabilities
Recovery Period	2 years
Maximum | IRMA long-term accounts receivable
Regulatory Assets and Liabilities
Recovery Period	2 years