Summary of significant accounting policies: (Details 6) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Regulatory assets and liabilities
Total Regulatory Assets	$ 352,902	$ 351,547
Total Regulatory Liabilities	129,985	164,000
Net Regulatory Assets	222,917	187,547
Other income
Capital credits from associated companies	1,919	2,095	2,096
Net revenue from Georgia Transmission and Georgia System Operations for shared Administrative and General costs	4,280	4,071	3,834
Miscellaneous other	214	38	(564)
Total	6,413	6,204	5,366
Accumulated retirement costs for other obligations
Regulatory assets and liabilities
Total Regulatory Liabilities	28,846	32,687
Net benefit of Rocky Mountain transactions | Rocky Mountain
Regulatory assets and liabilities
Total Regulatory Liabilities	4,303	47,783
Amortization period	30 years
Major maintenance sinking fund
Regulatory assets and liabilities
Total Regulatory Liabilities	30,948	28,524
Other regulatory liabilities
Regulatory assets and liabilities
Total Regulatory Liabilities	1,289	1,609
Other regulatory liabilities | Maximum
Regulatory assets and liabilities
Amortization period	8 years
Debt service adder
Regulatory assets and liabilities
Total Regulatory Liabilities	47,486	37,586
Premium and loss on required debt
Regulatory assets and liabilities
Total Regulatory Assets	86,319	98,538
Premium and loss on required debt | Maximum
Regulatory assets and liabilities
Amortization period	30 years
Amortization on capital leases
Regulatory assets and liabilities
Total Regulatory Assets	28,670	46,627
Outage costs
Regulatory assets and liabilities
Total Regulatory Assets	30,901	42,866
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
Interest rate swap termination fees
Regulatory assets and liabilities
Total Regulatory Assets	17,326	21,316
Asset retirement obligations
Regulatory assets and liabilities
Total Regulatory Assets	11,382	29,341
Depreciation expense
Regulatory assets and liabilities
Total Regulatory Assets	49,785	51,209
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
Interest rate options cost
Regulatory assets and liabilities
Total Regulatory Assets	75,716	30,735
Other regulatory assets
Regulatory assets and liabilities
Total Regulatory Assets	8,379	9,777
Other regulatory assets | Maximum
Regulatory assets and liabilities
Amortization period	36 years
Vogtle Units No. 3 and No. 4 training costs | Vogtle Units No.3 and No.4
Regulatory assets and liabilities
Total Regulatory Assets	23,030	17,602
Effects on net margin- Smith Energy Facility | Smith
Regulatory assets and liabilities
Total Regulatory Assets	21,394	3,536
Effects on net margin- Smith Energy Facility | Hawk Road
Regulatory assets and liabilities
Total Regulatory Liabilities	$ 17,113	$ 15,811