Debt: (Details) (USD $)	12 Months Ended																0 Months Ended
	Dec. 31, 2013 item	Dec. 31, 2012	Dec. 31, 2013 Long-term Debt and Capital Leases	Dec. 31, 2013 Long-term Debt	Dec. 31, 2013 FFB	Feb. 20, 2014 FFB Subsequent event	Dec. 31, 2013 CFC	Dec. 31, 2013 CoBank	Dec. 31, 2013 PCBs	Dec. 31, 2013 CREBs	Feb. 20, 2014 Term Loans Subsequent event	Dec. 31, 2013 Capital Leases	Dec. 31, 2013 Committed credit arrangements item	Dec. 31, 2013 Letters of credit	Dec. 31, 2013 Commercial paper	Feb. 20, 2014 Commercial paper Subsequent event	Mar. 01, 2013 Pollution control revenue bonds	Apr. 23, 2013 Pollution control revenue bonds	Dec. 31, 2013 Series of bonds bear interest at 2.40% per annum until April 1, 2020, due 2038 through 2040 Georgia
Debt
Number of commercial banks to whom unsecured notes is payable	3
Principal amount					$ 676,097,000	$ 725,000,000												$ 212,760,000
Repayment of outstanding debt											260,000,000					465,000,000
Interest rate (as a percent)																			2.40%
Refund of pollution control revenue bonds																	212,760,000
Maturities for long-term debt and capital lease obligations
2014			152,153,000	133,672,000	131,235,000		806,000	621,000		1,010,000		18,481,000
2015			293,173,000	271,898,000	135,792,000		848,000	698,000	133,550,000	1,010,000		21,275,000
2016			258,851,000	254,896,000	140,154,000		891,000	786,000	112,055,000	1,010,000		3,955,000
2017			144,497,000	140,093,000	137,261,000		937,000	885,000		1,010,000		4,404,000
2018			150,129,000	145,224,000	142,234,000		984,000	996,000		1,010,000		4,905,000
Weighted average interest rate for long-term debt and capital leases, excluding short-term borrowings classified as long-term	4.56%	4.73%
Maximum borrowing capacity													1,925,000,000	1,045,000,000
Available borrowing capacity														794,000,000
Number of separate facilities													6
Line of credit, amount outstanding															744,000,000
Letters of credit, amount outstanding														$ 251,000,000
Weighted average interest rate on short-term borrowings (as a percent)	0.29%	0.44%