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Derivative Instruments (Details) - GOLDEN STATE WATER COMPANY
MWh in Thousands, $ in Thousands
1 Months Ended 3 Months Ended 6 Months Ended
Dec. 31, 2014
Jun. 30, 2018
USD ($)
quote
Jun. 30, 2017
USD ($)
Jun. 30, 2018
USD ($)
MWh
quote
Jun. 30, 2017
USD ($)
Derivative Instruments, Gain (Loss) [Line Items]          
Description of derivative activity volume | MWh       149  
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   $ (2,625) $ (5,460) $ (2,941) $ (4,901)
Unrealized gain on purchased power contracts   915 967 1,231 408
Fair value at end of the period   $ (1,710) $ (4,493) $ (1,710) $ (4,493)
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