Selected Balance Sheet Data Balance Sheet Related Disclosures, Inventories (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Public Utilities, Inventory [Line Items]
Inventories	$ 156,900	$ 197,400
Materials and supplies
Public Utilities, Inventory [Line Items]
Inventories	62,400	61,900
Fuel
Public Utilities, Inventory [Line Items]
Inventories	64,500	69,500
Natural gas
Public Utilities, Inventory [Line Items]
Inventories	$ 30,000	$ 66,000