Investments: (Details 2) (USD $)	12 Months Ended
	Dec. 31, 2013 item	Dec. 31, 2012	Dec. 31, 2011
Investments:
Gross realized gains	$ 46,647,000	$ 28,959,000	$ 31,130,000
Gross realized losses	(21,685,000)	(18,175,000)	(16,215,000)
Proceeds from sales	605,364,000	666,481,000	1,064,377,000
Investment in associated companies
Investment in associated companies	66,437,000	60,770,000
Number of cooperative members	38
National Rural Utilities Cooperative Finance Corporation (CFC)
Investment in associated companies
Investment in associated companies	24,019,000	24,005,000
CoBank, ACB
Investment in associated companies
Investment in associated companies	2,517,000	2,931,000
CT Parts, LLC
Investment in associated companies
Investment in associated companies	8,853,000	7,128,000
Georgia Transmission Corporation
Investment in associated companies
Investment in associated companies	22,542,000	20,867,000
Number of cooperative members	38
Expenses incurred for transmission services, system operations services and administrative support services	27,599,000	26,035,000	25,128,000
Georgia System Operations Corporation
Investment in associated companies
Investment in associated companies	7,000,000	4,375,000
Number of cooperative members	37
Expenses incurred for transmission services, system operations services and administrative support services	20,354,000	18,870,000	17,793,000
Other
Investment in associated companies
Investment in associated companies	$ 1,506,000	$ 1,464,000