Electric plant, construction and related agreements: Summary of plant investments and related accumulated depreciation (Details) - USD ($)	Dec. 31, 2015	Dec. 31, 2014
Electric plant, construction and related agreements
Investment	$ 8,596,148,000	$ 8,345,241,000
Accumulated Depreciation	(3,925,838,000)	(3,762,690,000)
Total construction work in progress	2,868,669,000	2,374,392,000
Plant acquisition adjustments	196,000,000	196,000,000
Vogtle Units Number 1 And Number 2
Electric plant, construction and related agreements
Investment	2,869,142,000	2,805,310,000
Accumulated Depreciation	$ (1,674,431,000)	(1,634,062,000)
Ownership interest (as a percent)	30.00%
Vogtle Units Number 3 And Number 4
Electric plant, construction and related agreements
Investment	$ 22,557,000	18,267,000
Accumulated Depreciation	(891,000)	(637,000)
Total construction work in progress	$ 2,724,543,000	2,268,344,000
Ownership interest (as a percent)	30.00%
Hatch Units Number 1 And Number 2
Electric plant, construction and related agreements
Investment	$ 767,019,000	695,392,000
Accumulated Depreciation	$ (391,822,000)	(377,902,000)
Ownership interest (as a percent)	30.00%
Wansley Units Number 1 And Number 2
Electric plant, construction and related agreements
Investment	$ 499,180,000	474,740,000
Accumulated Depreciation	$ (163,266,000)	(157,543,000)
Ownership interest (as a percent)	30.00%
Scherer Unit Number 1
Electric plant, construction and related agreements
Investment	$ 1,086,435,000	1,067,777,000
Accumulated Depreciation	$ (332,493,000)	(318,990,000)
Ownership interest (as a percent)	60.00%
Doyle
Electric plant, construction and related agreements
Investment	$ 124,051,000	126,990,000
Accumulated Depreciation	$ (90,590,000)	(95,644,000)
Ownership interest (as a percent)	100.00%
Rocky Mountain Units Number 1, Number 2 And Number 3
Electric plant, construction and related agreements
Investment	$ 600,640,000	587,762,000
Accumulated Depreciation	$ (222,888,000)	(211,010,000)
Ownership interest (as a percent)	75.00%
Hartwell
Electric plant, construction and related agreements
Investment	$ 226,737,000	224,512,000
Accumulated Depreciation	$ (103,111,000)	(98,696,000)
Ownership interest (as a percent)	100.00%
Hawk Road
Electric plant, construction and related agreements
Investment	$ 243,517,000	242,293,000
Accumulated Depreciation	$ (70,832,000)	(65,931,000)
Ownership interest (as a percent)	100.00%
Talbot
Electric plant, construction and related agreements
Investment	$ 290,463,000	289,020,000
Accumulated Depreciation	$ (111,617,000)	(102,947,000)
Ownership interest (as a percent)	100.00%
Chattahoochee
Electric plant, construction and related agreements
Investment	$ 310,788,000	308,943,000
Accumulated Depreciation	$ (114,914,000)	(106,633,000)
Ownership interest (as a percent)	100.00%
Smith
Electric plant, construction and related agreements
Investment	$ 601,903,000	579,486,000
Accumulated Depreciation	$ (166,324,000)	(151,677,000)
Ownership interest (as a percent)	100.00%
Wansley
Electric plant, construction and related agreements
Investment	$ 3,582,000	3,582,000
Accumulated Depreciation	$ (3,513,000)	(3,377,000)
Ownership interest (as a percent)	30.00%
Transmission
Electric plant, construction and related agreements
Investment	$ 90,195,000	88,031,000
Accumulated Depreciation	(50,786,000)	(49,132,000)
Other production
Electric plant, construction and related agreements
Investment	116,396,000	112,431,000
Accumulated Depreciation	(73,442,000)	(63,427,000)
Scherer Unit Number 2
Electric plant, construction and related agreements
Investment	743,543,000	720,705,000
Accumulated Depreciation	$ (354,918,000)	(325,082,000)
Ownership interest (as a percent)	60.00%
Environmental and other generation improvements
Electric plant, construction and related agreements
Total construction work in progress	$ 143,723,000	104,600,000
Other Construction Work In Progress
Electric plant, construction and related agreements
Total construction work in progress	$ 403,000	$ 1,448,000