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Fair Value Measurement (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Long-term debt $ 510,903   $ 431,424
Unrealized loss on California Water Service Group stock 0 $ (667)  
Available for Sale Marketable Security, Shares Sold   17,660  
Proceeds from sale of California Water Service Group stock   $ 714  
Fees Incurred on Sale of Available for Sale Securities   2  
Gain on sale of California Water Service Group stock   (87)  
Available for Sales Securities, Tax on Realized Gain   (24)  
Debt and Equity Securities, Gain (Loss)   $ (63)  
Significant Observable Inputs      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Long-term debt, fair value $ 590,450   $ 490,148