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Derivative Instruments (Details)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2020
USD ($)
Dec. 31, 2019
USD ($)
Sep. 30, 2019
USD ($)
Sep. 30, 2020
USD ($)
MWh
Sep. 30, 2019
USD ($)
Commodity Contract          
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]          
Fair value at end of the period $ 2,500     $ 2,500  
GOLDEN STATE WATER COMPANY          
Derivative Instruments, Gain (Loss) [Line Items]          
Description of derivative activity volume | MWh       528,442  
GOLDEN STATE WATER COMPANY | Minimum | Commodity Contract          
Derivative Instruments, Gain (Loss) [Line Items]          
Term of derivative contract   3 years      
GOLDEN STATE WATER COMPANY | Maximum | Commodity Contract          
Derivative Instruments, Gain (Loss) [Line Items]          
Term of derivative contract   5 years      
Bear Valley Electric Service, Inc. | Commodity Contract          
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]          
Fair value at beginning of the period (3,827) $ (3,022) $ (267) $ (3,171) $ (311)
Unrealized gains (losses) on purchased power contracts 1,309   (2,755) 653 (2,711)
Fair value at end of the period $ (2,518) $ (3,171) $ (3,022) $ (2,518) $ (3,022)