Summary of significant accounting policies: - Accounting policies (Details)	3 Months Ended		12 Months Ended
	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2016 USD ($) item MW	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Business description
Number of electric distribution cooperative members | item			38
Summer planning reserve capacity of generating units (in megawatts) | MW			7,809
Number of people to whom energy is distributed on a retail basis by the entity's members | item			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 (deficit) margin
Effective income tax rate (as a percent)			0.00%	0.00%	0.00%
Balance at the beginning of the period			$ 58,000	$ 468,000	$ (549,000)
Unrealized gain (loss)			(234,000)	265,000	1,180,000
(Gain) reclassified to net margin			(194,000)	(675,000)	(163,000)
Balance at the end of the period	$ (370,000)	$ 58,000	$ (370,000)	$ 58,000	$ 468,000
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
Receivables
Member receivables	$ 136,552,000	$ 108,729,000	$ 136,552,000	$ 108,729,000
Electric plant
Construction rates for allowance of funds (as a percent)			4.61%	4.73%	4.97%
Restricted cash and investments
Guaranteed interest rate on deposit (as a percent)			5.00%	5.00%
Restricted cash and investments	468,179,000	387,961,000	$ 468,179,000	$ 387,961,000
Restricted cash and investments - long-term	221,122,000	134,690,000	221,122,000	134,690,000
Inventories
Fossil fuels inventories	57,289,000	104,086,000	57,289,000	104,086,000
Spare parts	$ 202,542,000	$ 195,166,000	$ 202,542,000	195,166,000
Related parties
Number of electric distribution cooperative members | item			38
Other income
Capital credits from associated companies			$ 1,679,000	1,859,000	$ 1,986,000
Net revenue from Georgia Transmission and Georgia System Operations for shared Administrative and General costs			6,553,000	6,278,000	4,944,000
Miscellaneous other			(5,561,000)	1,006,000	(310,000)
Total			$ 2,671,000	9,143,000	6,620,000
Smarr EMC
Business description
Summer planning reserve capacity of generating units (in megawatts) | MW			728
Georgia Transmission
Business description
Number of electric distribution cooperative members | item			38
Related parties
Number of electric distribution cooperative members | item			38
Expenses incurred for transmission services, system operations services and administrative support services			$ 27,399,000	28,172,000	27,893,000
Georgia System Operations Corporation
Business description
Number of electric distribution cooperative members | item			38
Related parties
Number of electric distribution cooperative members | item			38
Expenses incurred for transmission services, system operations services and administrative support services			$ 23,994,000	$ 22,616,000	$ 23,351,000