Investments: (Details 2) (USD $)	12 Months Ended
	Dec. 31, 2012 item	Dec. 31, 2011	Dec. 31, 2010
Investments:
Gross realized gains	$ 28,959,000	$ 31,130,000	$ 24,634,000
Gross realized losses	(18,175,000)	(16,215,000)	(10,395,000)
Proceeds from sales	666,481,000	1,064,377,000	622,124,000
Investment in associated companies
Investment in associated companies	60,770,000	57,626,000
Number of cooperative members	38
National Rural Utilities Cooperative Finance Corporation (CFC)
Investment in associated companies
Investment in associated companies	24,005,000	23,993,000
CoBank, ACB
Investment in associated companies
Investment in associated companies	2,931,000	3,345,000
CT Parts, LLC
Investment in associated companies
Investment in associated companies	7,128,000	5,633,000
Georgia Transmission Corporation
Investment in associated companies
Investment in associated companies	20,867,000	19,291,000
Number of cooperative members	38
Expenses incurred for transmission services, system operations services and administrative support services	26,035,000	25,128,000	25,491,000
Georgia System Operations Corporation
Investment in associated companies
Investment in associated companies	4,375,000	3,911,000
Number of cooperative members	37
Expenses incurred for transmission services, system operations services and administrative support services	18,870,000	17,793,000	16,848,000
Other
Investment in associated companies
Investment in associated companies	$ 1,464,000	$ 1,453,000
Smarr EMC
Investment in associated companies
Number of cooperative members	35
Number of combustion turbine facilities owned	2