Selected Balance Sheet Data Balance Sheet Related Disclosures, Inventories (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Public Utilities, Inventory [Line Items]
Inventories	$ 153,926	$ 214,489
Materials and supplies
Public Utilities, Inventory [Line Items]
Inventories	69,378	68,940
Fuel
Public Utilities, Inventory [Line Items]
Inventories	53,282	73,893
Natural gas
Public Utilities, Inventory [Line Items]
Inventories	$ 31,266	$ 71,656