Summary of significant accounting policies: (Details 6) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Regulatory assets and liabilities
Total Regulatory Assets	$ 331,108	$ 352,902
Total Regulatory Liabilities	158,789	129,985
Net Regulatory Assets	172,319	222,917
Other income
Capital credits from associated companies	1,954	1,919	2,095
Net revenue from Georgia Transmission and Georgia System Operations for shared Administrative and General costs	4,459	4,280	4,071
Miscellaneous other	(723)	214	38
Total	5,690	6,413	6,204
Accumulated retirement costs for other obligations
Regulatory assets and liabilities
Total Regulatory Liabilities	24,520	28,846
Effects on net margin- Hawk Road Energy Facility | Hawk Road
Regulatory assets and liabilities
Total Regulatory Liabilities	23,379	17,113
Major maintenance sinking fund
Regulatory assets and liabilities
Total Regulatory Liabilities	28,064	30,948
Debt service adder
Regulatory assets and liabilities
Total Regulatory Liabilities	57,223	47,486
Asset retirement obligations
Regulatory assets and liabilities
Total Regulatory Liabilities	19,508
Other regulatory liabilities
Regulatory assets and liabilities
Total Regulatory Liabilities	6,095	5,592
Other regulatory liabilities | Maximum
Regulatory assets and liabilities
Amortization period	18 years
Premium and loss on reacquired debt
Regulatory assets and liabilities
Total Regulatory Assets	82,499	86,319
Premium and loss on reacquired debt | Maximum
Regulatory assets and liabilities
Amortization period	30 years
Amortization on capital leases
Regulatory assets and liabilities
Total Regulatory Assets	16,124	28,670
Outage costs
Regulatory assets and liabilities
Total Regulatory Assets	35,155	30,901
Interest rate swap termination fees
Regulatory assets and liabilities
Total Regulatory Assets	13,336	17,326
Asset retirement obligations
Regulatory assets and liabilities
Total Regulatory Assets		11,382
Depreciation expense
Regulatory assets and liabilities
Total Regulatory Assets	48,362	49,785
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 No.3 and No.4
Regulatory assets and liabilities
Total Regulatory Assets	27,678	23,030
Interest rate options cost
Regulatory assets and liabilities
Total Regulatory Assets	38,984	75,716
Deferral of effects on net margin - Smith Energy Facility | Smith
Regulatory assets and liabilities
Total Regulatory Assets	63,491	21,394
Other regulatory assets
Regulatory assets and liabilities
Total Regulatory Assets	$ 5,479	$ 8,379
Other regulatory assets | Maximum
Regulatory assets and liabilities
Amortization period	35 years
Coal-fired outage costs | Minimum
Regulatory assets and liabilities
Amortization period	18 months
Coal-fired outage costs | Maximum
Regulatory assets and liabilities
Amortization period	36 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