Summary of significant accounting policies: (Details 2) (USD $)	12 Months Ended			0 Months Ended	3 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Apr. 05, 2012 Nuclear fuel disposal cost settlement Plant Hatch and Plant Vogtle	Jun. 30, 2012 Nuclear fuel disposal cost settlement Plant Hatch and Plant Vogtle
Receivables
Member receivables	$ 117,287,000	$ 109,673,000
Nuclear fuel cost
Nuclear fuel expense	86,828,000	81,723,000	74,814,000
Settlement amount, entity share				16,205,000
Settlement amount, all parties				54,017,000
Reduction of operating expenses					9,679,000
Reduction to plant in service					$ 6,526,000