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Derivative Instruments (Details) - Golden State Water Company
MWh in Thousands, $ in Thousands
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2018
USD ($)
quote
Sep. 30, 2017
USD ($)
Sep. 30, 2018
USD ($)
MWh
quote
Sep. 30, 2017
USD ($)
Dec. 31, 2015
Derivative Instruments, Gain (Loss) [Line Items]          
Description of derivative activity volume | MWh     123    
Commodity Contract          
Derivative Instruments, Gain (Loss) [Line Items]          
Number of broker quotes received to determine fair value of derivative instrument | quote 1   1    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]          
Fair value at beginning of the period $ (1,710) $ (4,493) $ (2,941) $ (4,901)  
Unrealized gain on purchased power contracts 567 656 1,798 1,064  
Fair value at end of the period $ (1,143) $ (3,837) $ (1,143) $ (3,837)  
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