Summary of significant accounting policies: Accounting policies (Details)	3 Months Ended		12 Months Ended
	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2015 USD ($) item MW	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Summary of significant accounting policies:
Number of electric distribution cooperative members | item			38
Number of people to whom energy is distributed on a retail basis by the entity's members | item			4,200,000
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 Loss Net Of Tax Roll Forward
Effective income tax rate (as a percent)			0.00%	0.00%	0.00%
Balance at the beginning of the period			$ 468,000	$ (549,000)	$ 903,000
Unrealized gain (loss)			265,000	1,180,000	(1,396,000)
(Gain) reclassified to net margin			(675,000)	(163,000)	(56,000)
Balance at the end of the period	$ 58,000	$ 468,000	$ 58,000	$ 468,000	$ (549,000)
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
Concentration
Number of programs allowing expense costs on current basis otherwise capitalized approved by Rural Utilities Service | item			2
Amounts billed to members			$ 25,375,000	$ 14,991,000	$ 13,962,000
Summer planning reserve capacity of generating units (in megawatts) | MW			7,067
Basis of accounting
Decrease in other adjustments to reconcile net margin to net cash provided by operating activities			$ (8,353,000)	(8,835,000)	(8,718,000)
Decrease in cash paid for property additions			$ (495,426,000)	$ (534,171,000)	$ (628,216,000)
Smarr EMC
Concentration
Summer planning reserve capacity of generating units (in megawatts) | MW			718
Total operating revenues | Revenues of members | Cobb EMC
Concentration
Operating revenues (as a percent)			13.10%	13.60%	13.20%
Total operating revenues | Revenues of members | Sawnee EMC
Concentration
Operating revenues (as a percent)			10.40%	9.50%	9.50%
Total operating revenues | Revenues of members | Jackson EMC
Concentration
Operating revenues (as a percent)			9.70%	10.40%	10.90%
Adjustments
Basis of accounting
Decrease in other adjustments to reconcile net margin to net cash provided by operating activities				$ 24,607,000
Decrease in cash paid for property additions				$ 24,607,000