Revenue Recognition (Details)	1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) item	Sep. 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	$ 15,000,000	$ 20,000,000	$ 15,000,000	$ 20,000,000	$ 15,000,000
Decrease in revenue requirement	5,000,000
Total		384,644,000	376,656,000	$ 1,124,211,000	1,092,195,000
Period in which payment is due following billing				1 month
Impairment losses		0	0	$ 0	0
Members
Revenue Recognition
Total		384,529,000	376,508,000	1,123,741,000	1,091,975,000
Receivables	$ 124,858,000	140,010,000	124,858,000	140,010,000	124,858,000		$ 126,211,000	$ 136,552,000
Non-Members
Revenue Recognition
Total		115,000	148,000	470,000	220,000
Capacity revenues | Members
Revenue Recognition
Total		219,597,000	220,347,000	691,649,000	687,725,000
Energy revenues | Members
Revenue Recognition
Total		$ 164,932,000	$ 156,161,000	$ 432,092,000	$ 404,250,000