Inventories (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Public Utilities, Inventory [Line Items]
Inventories	$ 280,700	$ 304,800
Materials and supplies
Public Utilities, Inventory [Line Items]
Inventories	177,800	175,600
Fuel
Public Utilities, Inventory [Line Items]
Inventories	96,100	103,200
Natural Gas
Public Utilities, Inventory [Line Items]
Inventories	$ 6,800	$ 26,000