Electric plant, construction and related agreements: (Details) (USD $)	Dec. 31, 2013	Dec. 31, 2012
Electric plant, construction and related agreements
Investment	$ 8,050,103,000	$ 7,506,707,000
Accumulated Depreciation	(3,615,375,000)	(3,472,087,000)
Total construction work in progress	2,212,224,000	2,240,920,000
Plant acquisition adjustments	196,000,000	196,000,000
Vogtle Units No. 1 & No. 2
Electric plant, construction and related agreements
Investment	2,778,941,000	2,745,800,000
Accumulated Depreciation	(1,602,942,000)	(1,566,133,000)
Ownership interest (as a percent)	30.00%
Vogtle Units No. 3 and No. 4
Electric plant, construction and related agreements
Investment	12,073,000	8,956,000
Accumulated Depreciation	(338,000)	(147,000)
Total construction work in progress	1,940,340,000	1,613,791,000
Ownership interest (as a percent)	30.00%
Hatch Units No. 1 & No. 2
Electric plant, construction and related agreements
Investment	679,373,000	645,722,000
Accumulated Depreciation	(367,757,000)	(361,554,000)
Ownership interest (as a percent)	30.00%
Wansley Units No. 1 & No. 2
Electric plant, construction and related agreements
Investment	441,590,000	441,340,000
Accumulated Depreciation	(149,594,000)	(147,890,000)
Ownership interest (as a percent)	30.00%
Scherer Unit No. 1
Electric plant, construction and related agreements
Investment	889,094,000	696,223,000
Accumulated Depreciation	(306,027,000)	(288,701,000)
Ownership interest (as a percent)	60.00%
Rocky Mountain Units No. 1, No. 2 & No. 3
Electric plant, construction and related agreements
Investment	587,312,000	585,133,000
Accumulated Depreciation	(199,386,000)	(187,763,000)
Ownership interest (as a percent)	75.00%
Hartwell
Electric plant, construction and related agreements
Investment	224,384,000	224,374,000
Accumulated Depreciation	(94,258,000)	(89,822,000)
Ownership interest (as a percent)	100.00%
Hawk Road
Electric plant, construction and related agreements
Investment	242,757,000	243,963,000
Accumulated Depreciation	(61,846,000)	(56,593,000)
Ownership interest (as a percent)	100.00%
Talbot
Electric plant, construction and related agreements
Investment	281,271,000	281,009,000
Accumulated Depreciation	(94,399,000)	(86,000,000)
Ownership interest (as a percent)	100.00%
Chattahoochee
Electric plant, construction and related agreements
Investment	306,953,000	301,246,000
Accumulated Depreciation	(97,392,000)	(88,272,000)
Ownership interest (as a percent)	100.00%
Smith
Electric plant, construction and related agreements
Investment	561,704,000	553,942,000
Accumulated Depreciation	(137,626,000)	(123,575,000)
Ownership interest (as a percent)	100.00%
Wansley
Electric plant, construction and related agreements
Investment	3,636,000	3,624,000
Accumulated Depreciation	(3,309,000)	(3,161,000)
Ownership interest (as a percent)	30.00%
Transmission plant
Electric plant, construction and related agreements
Investment	86,088,000	84,801,000
Accumulated Depreciation	(47,286,000)	(45,381,000)
Other
Electric plant, construction and related agreements
Investment	111,887,000	109,461,000
Accumulated Depreciation	(62,904,000)	(67,415,000)
Doyle
Electric plant, construction and related agreements
Investment	126,990,000	126,990,000
Accumulated Depreciation	(92,944,000)	(82,546,000)
Ownership interest (as a percent)	100.00%
Scherer Unit No. 2
Electric plant, construction and related agreements
Investment	716,050,000	454,123,000
Accumulated Depreciation	(297,367,000)	(277,134,000)
Ownership interest (as a percent)	60.00%
Environmental and other generation improvements
Electric plant, construction and related agreements
Total construction work in progress	270,446,000	626,264,000
Other
Electric plant, construction and related agreements
Total construction work in progress	$ 1,438,000	$ 865,000