Selected Balance Sheet Data, Inventory (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Public Utilities, Inventory [Line Items]
Inventories	$ 18,385	$ 17,758
Materials and supplies
Public Utilities, Inventory [Line Items]
Inventories	6,770	6,916
Fuel
Public Utilities, Inventory [Line Items]
Inventories	3,718	3,866
Natural gas
Public Utilities, Inventory [Line Items]
Inventories	$ 7,897	$ 6,976