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Balance Sheet Components Materials and Supplies, Fuel Stock and Stored Natural Gas (Details) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Balance Sheet Components [Abstract]    
Inventory, Raw Materials and Supplies, Gross $ 44,335 $ 41,493
Other Inventory, Gross 5,958 4,843
Energy Related Inventory, Gas Stored Underground 6,608 11,739
Inventory, Net $ 56,901 $ 58,075