Selected Balance Sheet Data Balance Sheet Related Disclosures, Inventories (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Public Utilities, Inventory [Line Items]
Inventories	$ 205,640	$ 202,220
Materials and supplies
Public Utilities, Inventory [Line Items]
Inventories	69,051	66,161
Fuel
Public Utilities, Inventory [Line Items]
Inventories	56,247	66,429
Natural gas
Public Utilities, Inventory [Line Items]
Inventories	$ 80,342	$ 69,630