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Derivative Instruments (Details) - GOLDEN STATE WATER COMPANY
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2019
USD ($)
MWh
quote
Mar. 31, 2018
USD ($)
Dec. 31, 2015
Derivative Instruments, Gain (Loss) [Line Items]      
Description of derivative activity volume | MWh 70,300    
Commodity Contract      
Derivative Instruments, Gain (Loss) [Line Items]      
Number of broker quotes received to determine fair value of derivative instrument | quote 1    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]      
Fair value at beginning of the period $ (311) $ (2,941)  
Unrealized gain (loss) on purchased power contracts (25) 316  
Fair value at end of the period $ (336) $ (2,625)  
Minimum | Commodity Contract      
Derivative Instruments, Gain (Loss) [Line Items]      
Term of derivative contract     3 years
Maximum | Commodity Contract      
Derivative Instruments, Gain (Loss) [Line Items]      
Term of derivative contract     5 years