Revenue Recognition (Details)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) item	Jun. 30, 2017 USD ($)	Dec. 31, 2018	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
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
Achieved margins for interest ratio						1.14
Refund liability	$ 5,650,000	$ 5,750,000	$ 5,650,000	$ 5,750,000
Total	365,921,000	361,369,000	$ 739,567,000	715,539,000
Period in which payment is due following billing			1 month
Impairment losses	0	0	$ 0	0
Members
Revenue Recognition
Total	365,811,000	361,323,000	739,212,000	715,467,000
Receivables	144,865,000	132,703,000	144,865,000	132,703,000		$ 126,211,000	$ 136,552,000
Non-Members
Revenue Recognition
Total	110,000	46,000	355,000	72,000
Capacity revenues | Members
Revenue Recognition
Total	231,571,000	229,946,000	472,052,000	467,377,000
Energy revenues | Members
Revenue Recognition
Total	$ 134,240,000	$ 131,377,000	$ 267,160,000	$ 248,090,000