Selected Balance Sheet Data, Inventory (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Public Utilities, Inventory [Line Items]
Inventories	$ 17,758	$ 18,309
Materials and supplies
Public Utilities, Inventory [Line Items]
Inventories	6,916	6,582
Fuel
Public Utilities, Inventory [Line Items]
Inventories	3,866	4,743
Natural gas
Public Utilities, Inventory [Line Items]
Inventories	$ 6,976	$ 6,984