REGULATORY ASSETS AND LIABILITIES - Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Regulatory Assets and Liabilities
Regulatory Assets	$ 325,376	$ 433,322
Regulatory Liabilities	180,718	211,413
Future tax benefits due to customers
Regulatory Assets and Liabilities
Regulatory Liabilities	151,011	194,501
Retiree group health
Regulatory Assets and Liabilities
Regulatory Liabilities	18,472	0
HCBA
Regulatory Assets and Liabilities
Regulatory Liabilities	5,320	4,271
Conservation program
Regulatory Assets and Liabilities
Regulatory Liabilities	3,837	2,742
Net WRAM and MCBA long-term payable
Regulatory Assets and Liabilities
Regulatory Liabilities	479	211
1,2,3 trichloropropane settlement proceeds
Regulatory Assets and Liabilities
Regulatory Liabilities	0	8,426
Other regulatory liabilities
Regulatory Assets and Liabilities
Regulatory Liabilities	1,599	1,262
Pension and retiree group health
Regulatory Assets and Liabilities
Regulatory Assets	59,588	208,321
Property-related temporary differences (tax benefits flowed through to customers)
Regulatory Assets and Liabilities
Regulatory Assets	120,365	104,931
Other accrued benefits
Regulatory Assets and Liabilities
Regulatory Assets	21,692	20,030
Net WRAM and MCBA long-term accounts receivable
Regulatory Assets and Liabilities
Regulatory Assets	33,136	25,465
Asset retirement obligations, net
Regulatory Assets and Liabilities
Regulatory Assets	21,110	19,567
IRMA long-term accounts receivable
Regulatory Assets and Liabilities
Regulatory Assets	$ 14,705	4,642
Tank coating
Regulatory Assets and Liabilities
Recovery Period	10 years
Regulatory Assets	$ 14,018	13,535
Recoverable property losses
Regulatory Assets and Liabilities
Recovery Period	9 years
Regulatory Assets	$ 4,531	5,000
PCBA
Regulatory Assets and Liabilities
Recovery Period	1 year
Regulatory Assets	$ 19,647	21,465
Other components of net periodic benefit cost
Regulatory Assets and Liabilities
Regulatory Assets	$ 6,736	5,145
General district balancing account receivable
Regulatory Assets and Liabilities
Recovery Period	1 year
Regulatory Assets	$ 1,830	425
Other regulatory assets
Regulatory Assets and Liabilities
Regulatory Assets	$ 2,708	4,009
Low Income Rate Assistance And Rate Support Fund Accounts Receivable
Regulatory Assets and Liabilities
Recovery Period	1 year
Regulatory Assets	$ 5,310	$ 787
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