Inventories (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Public Utilities, Inventory [Line Items]
Inventories	$ 285,700	$ 304,800
Materials and supplies
Public Utilities, Inventory [Line Items]
Inventories	179,900	175,600
Fuel
Public Utilities, Inventory [Line Items]
Inventories	95,100	103,200
Natural Gas
Public Utilities, Inventory [Line Items]
Inventories	$ 10,700	$ 26,000