Summary of significant accounting policies: (Details) (USD $)	3 Months Ended		12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 item MW	Dec. 31, 2011	Dec. 31, 2010
Business description
Number of electric distribution cooperative members			38
Nameplate capacity of generating units (in megawatts)			6,844
Number of people to whom energy is distributed on a retail basis by the entity's members			4,100,000
Patronage capital and membership fees
Membership fees paid by members			$ 195
Minimum equity as a percentage of total long-term debt and equities for distributions of patronage capital			20.00%
Maximum percentage of aggregate net margins in which specified percentage of total long-term debt and equities cannot exceed on or after distributions expended			35.00%
Minimum equity as a percentage of total long-term debt and equities after distributions of patronage capital			30.00%
Accumulated other comprehensive margin (deficit)
Balance at the beginning of the period			618,000	(469,000)	(1,253,000)
Unrealized gain			285,000	1,087,000	784,000
Balance at the end of the period	903,000	618,000	903,000	618,000	(469,000)
Margin policy
Minimum margins for interest ratio under the first mortgage indenture			1.10
Margins for interest ratio	1.14	1.14	1.14	1.14	1.14
Operating revenues
Number of plans allowing expense costs on current basis otherwise capitalized approved by Rural Utilities Service			2
Amounts billed to members			$ 26,149,000	$ 5,436,000
Total operating revenues | Revenues of members | Cobb EMC
Operating revenues
Operating revenues (as a percent)			12.80%	12.50%	14.50%
Total operating revenues | Revenues of members | Jackson EMC
Operating revenues
Operating revenues (as a percent)			11.90%	10.90%	11.60%
Total operating revenues | Revenues of members | Sawnee EMC
Operating revenues
Operating revenues (as a percent)					10.60%