Revenue Recognition (Details)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) item	Jun. 30, 2018 USD ($)	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	$ 4,500,000	$ 5,650,000	$ 4,500,000	$ 5,650,000
Operating revenues	358,860,000	365,921,000	715,460,000	739,567,000
Members
Revenue Recognition
Operating revenues	358,736,000	365,811,000	715,206,000	739,212,000
Non-Members
Revenue Recognition
Operating revenues	124,000	110,000	254,000	355,000
Capacity revenues | Members
Revenue Recognition
Operating revenues	235,049,000	231,571,000	481,035,000	472,052,000
Energy revenues | Members
Revenue Recognition
Operating revenues	$ 123,687,000	$ 134,240,000	$ 234,171,000	$ 267,160,000