Other - (Schedule of Operating Revenues) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Other [Abstract]
Sales to member distribution cooperatives, excluding renewable energy credit sales	$ 184,734	$ 204,165
Renewable energy credit sales to member distribution cooperatives	9
Total sales to member distribution cooperatives	184,743	204,165
Sales to non-members, excluding renewable energy credit sales	6,191	4,477
Renewable energy credit sales to non-members	183	104
Total sales to non-members	6,374	4,581
Total operating revenues	$ 191,117	$ 208,746
Type of Revenue [Extensible List]	us-gaap:ElectricityMember	us-gaap:ElectricityMember