Selected Balance Sheet Data, Inventory (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Public Utilities, Inventory [Line Items]
Inventories	$ 11,655	$ 17,758
Materials and supplies
Public Utilities, Inventory [Line Items]
Inventories	7,015	6,916
Fuel
Public Utilities, Inventory [Line Items]
Inventories	3,842	3,866
Natural gas
Public Utilities, Inventory [Line Items]
Inventories	$ 798	$ 6,976