Summary of significant accounting policies: (Details 4) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012 Hatch Unit No. 1	Dec. 31, 2012 Hatch Unit No. 2	Dec. 31, 2012 Vogtle Unit No. 1	Dec. 31, 2012 Vogtle Unit No. 2
Nuclear decommissioning
Estimated costs based on site study		$ 186,000	$ 252,000	$ 182,000	$ 241,000
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)	7.50%