Summary of significant accounting policies: Regulatory assets and liabilities (Details)	12 Months Ended
	Dec. 31, 2015 USD ($) item	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 530,254,000	$ 484,049,000
Total Regulatory Liabilities	166,967,000	194,073,000
Net Regulatory Assets	$ 363,287,000	289,976,000
Related parties
Number of electric distribution cooperative members | item	38
Other income
Capital credits from associated companies	$ 1,859,000	1,986,000	$ 1,954,000
Net revenue from Georgia Transmission and Georgia System Operations for shared Administrative and General costs	6,278,000	4,944,000	4,459,000
Miscellaneous other	1,006,000	(310,000)	(723,000)
Total	$ 9,143,000	6,620,000	5,690,000
Georgia Transmission
Related parties
Number of electric distribution cooperative members | item	38
Expenses incurred for transmission services, system operations services and administrative support services	$ 28,172,000	27,893,000	27,599,000
Georgia System Operations Corporation
Related parties
Number of electric distribution cooperative members | item	38
Expenses incurred for transmission services, system operations services and administrative support services	$ 22,616,000	27,893,000	$ 27,599,000
Accumulated retirement costs for other obligations
Regulatory Assets and Liabilities
Total Regulatory Liabilities	8,910,000	18,559,000
Deferral of effects on net margin- Hawk Road Energy Facility | Hawk Road
Regulatory Assets and Liabilities
Total Regulatory Liabilities	20,775,000	29,867,000
Major maintenance reserve
Regulatory Assets and Liabilities
Total Regulatory Liabilities	22,422,000	23,427,000
Amortization on Capital Leases
Regulatory Assets and Liabilities
Total Regulatory Liabilities	26,502,000	21,693,000
Deferred debt service adder
Regulatory Assets and Liabilities
Total Regulatory Liabilities	76,334,000	66,754,000
Asset retirement obligations.
Regulatory Assets and Liabilities
Total Regulatory Liabilities	8,316,000	28,870,000
Other regulatory liabilities
Regulatory Assets and Liabilities
Total Regulatory Liabilities	$ 3,708,000	4,903,000
Other regulatory liabilities | Maximum
Regulatory Assets and Liabilities
Amortization period	11 years
Premium and loss on reacquired debt
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 61,916,000	71,731,000
Premium and loss on reacquired debt | Maximum
Regulatory Assets and Liabilities
Amortization period	28 years
Amortization on capital leases
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 30,253,000	27,829,000
Outage costs
Regulatory Assets and Liabilities
Total Regulatory Assets	42,027,000	45,795,000
Interest rate swap termination fees
Regulatory Assets and Liabilities
Total Regulatory Assets	5,355,000	9,345,000
Depreciation expense
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 45,514,000	46,938,000
Depreciation expense | Plant Vogtle
Regulatory Assets and Liabilities
Operating license expected extension period for Plant Vogtle	20 years
Operating license, expected extension period, for Plant Vogtle	40 years
Deferred charges related to Vogtle Units No. 3 and No. 4 training costs | Vogtle Units Number 3 And Number 4
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 37,646,000	32,501,000
Interest rate options cost
Regulatory Assets and Liabilities
Total Regulatory Assets	102,554,000	98,671,000
Deferral of effects on net margin - Smith Energy Facility | Smith
Regulatory Assets and Liabilities
Total Regulatory Assets	178,343,000	128,666,000
Other regulatory assets
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 26,646,000	$ 22,573,000
Other regulatory assets | Maximum
Regulatory Assets and Liabilities
Amortization period	34 years
Coal-fired outage costs
Regulatory Assets and Liabilities
Amortization period	24 months
Nuclear refueling outage costs | Minimum
Regulatory Assets and Liabilities
Amortization period	18 months
Nuclear refueling outage costs | Maximum
Regulatory Assets and Liabilities
Amortization period	24 months