Select Balance Sheet Information - Schedule of Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Jun. 30, 2022	Dec. 31, 2021
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, net of accumulated depreciation	$ 2,149,919	$ 2,124,605
Electric Plant
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	2,882,613	2,833,371
Less Accumulated Depreciation and Amortization	843,296	817,302
Property, Plant and Equipment, net of accumulated depreciation	2,039,317	2,016,069
Nonelectric Property, Plant and Equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	297,606	290,561
Less Accumulated Depreciation and Amortization	187,004	182,025
Property, Plant and Equipment, net of accumulated depreciation	110,602	108,536
Electric Plant in Service | Electric Plant
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	2,785,850	2,758,445
Construction Work in Progress | Electric Plant
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	96,763	74,926
Construction Work in Progress | Nonelectric Property, Plant and Equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	18,483	16,611
Nonelectric Property, Plant and Equipment in Service | Nonelectric Property, Plant and Equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 279,123	$ 273,950