Summary of significant accounting policies: (Details 4) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012
Nuclear decommissioning trust fund
Escalation rate assumed for labor, materials and equipment cost (as a percent)	2.40%
Earning rate assumed on decommissioning trust fund (as a percent)	6.00%
Average annualized return produced by the nuclear decommissioning trust fund (as a percent)	7.10%
Hatch Unit No. 1
Nuclear decommissioning trust fund
Estimated costs based on site study	186,000
Hatch Unit No. 2
Nuclear decommissioning trust fund
Estimated costs based on site study	252,000
Vogtle Unit No. 1
Nuclear decommissioning trust fund
Estimated costs based on site study	182,000
Vogtle Unit No. 2
Nuclear decommissioning trust fund
Estimated costs based on site study	241,000