Summary Of Significant Accounting Policies - (Schedule of Operating Revenues) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Accounting Policies [Abstract]
Energy revenues	$ 328,045	$ 375,714	$ 471,767
Renewable energy credits	36	33	26
Demand revenues	398,819	395,067	426,678
Total revenues from sales to member distribution cooperatives	726,900	770,814	898,471
Energy revenues	45,255	31,431	29,483
Renewable energy credits	8,485	5,438	4,672
Demand revenues		21	56
Total revenues from sales to non-members	53,740	36,890	34,211
Total operating revenues	$ 780,640	$ 807,704	$ 932,682
Revenue, Product and Service [Extensible Enumeration]	us-gaap:ElectricityMember	us-gaap:ElectricityMember	us-gaap:ElectricityMember