Electric plant, construction and related agreements: (Details) (USD $)	Dec. 31, 2012	Dec. 31, 2011
Electric plant, construction and related agreements
Investment	$ 7,506,707,000	$ 7,335,866,000
Accumulated Depreciation	(3,472,087,000)	(3,328,585,000)
Total construction work in progress	2,240,920,000	1,784,264,000
Plant acquisition adjustments	196,000,000	198,000,000
Vogtle Units No. 1 & No. 2
Electric plant, construction and related agreements
Investment	2,745,800,000	2,726,129,000
Accumulated Depreciation	(1,566,133,000)	(1,528,841,000)
Ownership interest (as a percent)	30.00%
Vogtle Units No. 3 and No. 4
Electric plant, construction and related agreements
Investment	8,956,000	0
Accumulated Depreciation	(147,000)	0
Total construction work in progress	1,613,791,000	1,287,649,000
Ownership interest (as a percent)	30.00%
Hatch Units No. 1 & No. 2
Electric plant, construction and related agreements
Investment	645,722,000	603,865,000
Accumulated Depreciation	(361,554,000)	(369,189,000)
Ownership interest (as a percent)	30.00%
Wansley Units No. 1 & No. 2
Electric plant, construction and related agreements
Investment	441,340,000	394,596,000
Accumulated Depreciation	(147,890,000)	(129,994,000)
Ownership interest (as a percent)	30.00%
Scherer Unit No. 1
Electric plant, construction and related agreements
Investment	696,223,000	663,583,000
Accumulated Depreciation	(288,701,000)	(269,525,000)
Ownership interest (as a percent)	60.00%
Rocky Mountain Units No. 1, No. 2 & No. 3
Electric plant, construction and related agreements
Investment	585,133,000	584,569,000
Accumulated Depreciation	(187,763,000)	(176,356,000)
Ownership interest (as a percent)	75.00%
Hartwell
Electric plant, construction and related agreements
Investment	224,374,000	223,936,000
Accumulated Depreciation	(89,822,000)	(85,337,000)
Ownership interest (as a percent)	100.00%
Hawk Road
Electric plant, construction and related agreements
Investment	243,963,000	241,587,000
Accumulated Depreciation	(56,593,000)	(51,253,000)
Ownership interest (as a percent)	100.00%
Talbot
Electric plant, construction and related agreements
Investment	281,009,000	280,413,000
Accumulated Depreciation	(86,000,000)	(77,616,000)
Ownership interest (as a percent)	100.00%
Chattahoochee
Electric plant, construction and related agreements
Investment	301,246,000	299,117,000
Accumulated Depreciation	(88,272,000)	(79,285,000)
Ownership interest (as a percent)	100.00%
Smith
Electric plant, construction and related agreements
Investment	553,942,000	554,158,000
Accumulated Depreciation	(123,575,000)	(109,741,000)
Ownership interest (as a percent)	100.00%
Wansley
Electric plant, construction and related agreements
Investment	3,624,000	3,684,000
Accumulated Depreciation	(3,161,000)	(3,084,000)
Ownership interest (as a percent)	30.00%
Transmission plant
Electric plant, construction and related agreements
Investment	84,801,000	78,470,000
Accumulated Depreciation	(45,381,000)	(41,539,000)
Other
Electric plant, construction and related agreements
Investment	109,461,000	106,083,000
Accumulated Depreciation	(67,415,000)	(64,433,000)
Doyle
Electric plant, construction and related agreements
Investment	126,990,000	126,990,000
Accumulated Depreciation	(82,546,000)	(80,030,000)
Ownership interest (as a percent)	100.00%
Scherer Unit No. 2
Electric plant, construction and related agreements
Investment	454,123,000	448,686,000
Accumulated Depreciation	(277,134,000)	(262,362,000)
Ownership interest (as a percent)	60.00%
Environmental and other generation improvements
Electric plant, construction and related agreements
Total construction work in progress	626,264,000	495,597,000
Other
Electric plant, construction and related agreements
Total construction work in progress	$ 865,000	$ 1,018,000