Selected Balance Sheet Data, Inventory (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Public Utilities, Inventory [Line Items]
Inventories	$ 20,107	$ 18,309
Materials and supplies
Public Utilities, Inventory [Line Items]
Inventories	7,417	6,582
Fuel
Public Utilities, Inventory [Line Items]
Inventories	3,898	4,743
Natural gas
Public Utilities, Inventory [Line Items]
Inventories	$ 8,792	$ 6,984