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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, 2017
USD ($)
quote
Sep. 30, 2016
USD ($)
Sep. 30, 2017
USD ($)
MWh
quote
Sep. 30, 2016
USD ($)
Dec. 31, 2015
USD ($)
Derivative Instruments, Gain (Loss) [Line Items]          
Description of derivative activity volume | MWh     245    
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 $ (4,493) $ (4,933) $ (4,901) $ (7,053)  
Unrealized gain (loss) on purchased power contracts 656 (648) 1,064 1,472  
Fair value at end of the period $ (3,837) $ (5,581) $ (3,837) $ (5,581) $ (7,053)
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