Summary of significant accounting policies: - Concentrations (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Total operating revenues | Revenues of members | Jackson EMC
Concentration
Operating revenues (as a percent)	14.30%	9.70%	10.40%
Total operating revenues | Revenues of members | Cobb EMC
Concentration
Operating revenues (as a percent)	13.70%	13.10%	13.60%
Total operating revenues | Revenues of members | Sawnee EMC
Concentration
Operating revenues (as a percent)	10.50%	10.40%	9.50%
Vogtle Units Number 3 And Number 4
Concentration
Amounts billed to members	$ 16,096,000	$ 7,630,000	$ 6,182,000
Smith
Concentration
Amounts billed to members		$ 17,745,000	$ 8,809,000