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Fair Value of Financial Instruments: (Details) - USD ($)
$ in Thousands
Jun. 30, 2016
Dec. 31, 2015
GSWC | Carrying Amount    
Financial liabilities:    
Long-term debt-GSWC $ 325,684 $ 325,853
GSWC | Fair Value    
Financial liabilities:    
Long-term debt-GSWC 442,389 $ 403,844
Mutual Funds | Level 1    
Fair value of financial instruments    
Investments $ 10,300