Summary of significant accounting policies: (Details) (USD $)	3 Months Ended		12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2013 item MW	Dec. 31, 2012	Dec. 31, 2011
Summary of significant accounting policies:
Number of electric distribution cooperative members			38
Summer planning reserve capacity of generating units (in megawatts)			7,035
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			$ 190
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)
Effective income tax rate (as a percent)			0.00%	0.00%	0.00%
Balance at the beginning of the period			903,000	618,000	(469,000)
Unrealized gain (loss)			(1,396,000)	770,000	1,461,000
(Gain) loss reclassified to net margin			(56,000)	(485,000)	(374,000)
Balance at the end of the period	(549,000)	903,000	(549,000)	903,000	618,000
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 programs allowing expense costs on current basis otherwise capitalized approved by Rural Utilities Service			2
Amounts billed to members			$ 13,962,000	$ 26,149,000	$ 5,436,000
Total operating revenues | Revenues of members | Cobb EMC
Operating revenues
Operating revenues (as a percent)			13.20%	12.80%	12.50%
Total operating revenues | Revenues of members | Jackson EMC
Operating revenues
Operating revenues (as a percent)			10.90%	11.90%	10.90%