Revenue Recognition (Details)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) item	Sep. 30, 2018 USD ($)	Dec. 31, 2019
Revenue Recognition
Number of electric distribution cooperative members | item			38
Number of services provided | item			2
Margins for interest ratio			1.10
Targeted margins for interest ratio					1.14
Refund liability	$ 7,700,000	$ 20,000,000	$ 7,700,000	$ 20,000,000
Operating revenues	$ 382,623,000	$ 384,644,000	$ 1,098,083,000	$ 1,124,211,000
Member energy requirements supplied	57.00%	57.00%	56.00%	58.00%
Members
Revenue Recognition
Operating revenues	$ 382,548,000	$ 384,529,000	$ 1,097,754,000	$ 1,123,741,000
Non-Members
Revenue Recognition
Operating revenues	75,000	115,000	329,000	470,000
Capacity revenues | Members
Revenue Recognition
Operating revenues	231,270,000	219,597,000	712,305,000	691,649,000
Energy revenues | Members
Revenue Recognition
Operating revenues	$ 151,278,000	$ 164,932,000	$ 385,449,000	$ 432,092,000