Electric plant, construction and related agreements: - Summary of plant investments and related accumulated depreciation (Details) - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Electric plant, construction and related agreements
Investment	$ 8,786,839,000	$ 8,596,148,000
Accumulated Depreciation	(4,115,339,000)	(3,925,838,000)
Total construction work in progress	3,228,214,000	2,868,669,000
Plant acquisition adjustments	197,000,000	197,000,000
Vogtle Units Number 1 And Number 2
Electric plant, construction and related agreements
Investment	2,885,559,000	2,869,142,000
Accumulated Depreciation	$ (1,712,642,000)	(1,674,431,000)
Ownership interest (as a percent)	30.00%
Vogtle Units Number 3 And Number 4
Electric plant, construction and related agreements
Investment	$ 36,163,000	22,557,000
Accumulated Depreciation	(1,567,000)	(891,000)
Total construction work in progress	$ 3,069,476,000	2,724,543,000
Ownership interest (as a percent)	30.00%
Hatch Units Number 1 And Number 2
Electric plant, construction and related agreements
Investment	$ 809,971,000	767,019,000
Accumulated Depreciation	$ (407,400,000)	(391,822,000)
Ownership interest (as a percent)	30.00%
Wansley Units Number 1 And Number 2
Electric plant, construction and related agreements
Investment	$ 577,781,000	499,180,000
Accumulated Depreciation	$ (190,974,000)	(163,266,000)
Ownership interest (as a percent)	30.00%
Scherer Unit Number 1
Electric plant, construction and related agreements
Investment	$ 1,083,772,000	1,086,435,000
Accumulated Depreciation	$ (368,948,000)	(332,493,000)
Ownership interest (as a percent)	60.00%
Doyle
Electric plant, construction and related agreements
Investment	$ 135,849,000	124,051,000
Accumulated Depreciation	$ (102,642,000)	(90,590,000)
Ownership interest (as a percent)	100.00%
Rocky Mountain Units Number 1, Number 2 And Number 3
Electric plant, construction and related agreements
Investment	$ 607,742,000	600,640,000
Accumulated Depreciation	$ (234,765,000)	(222,888,000)
Ownership interest (as a percent)	75.00%
Hartwell
Electric plant, construction and related agreements
Investment	$ 227,878,000	226,737,000
Accumulated Depreciation	$ (104,342,000)	(103,111,000)
Ownership interest (as a percent)	100.00%
Hawk Road
Electric plant, construction and related agreements
Investment	$ 250,595,000	243,517,000
Accumulated Depreciation	$ (69,984,000)	(70,832,000)
Ownership interest (as a percent)	100.00%
Talbot
Electric plant, construction and related agreements
Investment	$ 290,790,000	290,463,000
Accumulated Depreciation	$ (119,874,000)	(111,617,000)
Ownership interest (as a percent)	100.00%
Chattahoochee
Electric plant, construction and related agreements
Investment	$ 313,693,000	310,788,000
Accumulated Depreciation	$ (123,946,000)	(114,914,000)
Ownership interest (as a percent)	100.00%
Smith
Electric plant, construction and related agreements
Investment	$ 614,453,000	601,903,000
Accumulated Depreciation	$ (176,701,000)	(166,324,000)
Ownership interest (as a percent)	100.00%
Wansley
Electric plant, construction and related agreements
Investment	$ 3,582,000	3,582,000
Accumulated Depreciation	$ (3,569,000)	(3,513,000)
Ownership interest (as a percent)	30.00%
Transmission
Electric plant, construction and related agreements
Investment	$ 92,085,000	90,195,000
Accumulated Depreciation	(53,251,000)	(50,786,000)
Other production
Electric plant, construction and related agreements
Investment	99,644,000	116,396,000
Accumulated Depreciation	(61,356,000)	(73,442,000)
Scherer Unit Number 2
Electric plant, construction and related agreements
Investment	757,282,000	743,543,000
Accumulated Depreciation	$ (383,378,000)	(354,918,000)
Ownership interest (as a percent)	60.00%
Environmental and other generation improvements
Electric plant, construction and related agreements
Total construction work in progress	$ 158,181,000	143,723,000
Other Construction Work In Progress
Electric plant, construction and related agreements
Total construction work in progress	$ 557,000	$ 403,000