Selected Balance Sheet Data, Inventory (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Public Utilities, Inventory [Line Items]
Inventories	$ 11,700	$ 17,100
Materials and supplies
Public Utilities, Inventory [Line Items]
Inventories	7,000	6,700
Fuel
Public Utilities, Inventory [Line Items]
Inventories	3,600	3,800
Natural gas
Public Utilities, Inventory [Line Items]
Inventories	$ 1,100	$ 6,600