CONSOLIDATED STATEMENTS OF CAPITALIZATION (Parenthetical)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Federal Financing Bank | First mortgage notes payable to the Federal Financing Bank at interest rates varying from 1.84% to 8.43% (average rate of 4.09% at December 31, 2016) due in quarterly installments through 2043
Secured Long-term debt:
Interest rate, minimum (as a percent)	1.84%	1.84%
Interest rate, maximum (as a percent)	8.43%	8.43%
Interest rate, average rate (as a percent)	4.09%
Federal Financing Bank | First mortgage notes payable to the Federal Financing Bank at interest rates varying from 2.51% to 3.87% (average rate of 3.36% at December 31, 2016) due in quarterly installments through 2044
Secured Long-term debt:
Interest rate, minimum (as a percent)	2.51%	2.51%
Interest rate, maximum (as a percent)	3.87%	3.87%
Interest rate, average rate (as a percent)	3.36%
National Rural Utilities Cooperative Finance Corporation | First mortgage notes payable to National Rural Utilities Cooperative Finance Corporation at interest rates varying from 4.35% to 4.90% (average rate of 4.60% at December 31, 2016) due in quarterly installments through 2020
Secured Long-term debt:
Interest rate, minimum (as a percent)	4.35%	4.35%
Interest rate, maximum (as a percent)	4.90%	4.90%
Interest rate, average rate (as a percent)	4.60%
Public | First mortgage bonds payable: Series 2006 First Mortgage Bonds, 5.534%, due 2031 through 2035
Secured Long-term debt:
Interest rate (as a percent)	5.534%	5.534%
Public | First mortgage bonds payable: Series 2007 First Mortgage Bonds, 6.191%, due 2024 through 2031
Secured Long-term debt:
Interest rate (as a percent)	6.191%	6.191%
Public | First mortgage bonds payable: Series 2009A First Mortgage Bonds, 6.10%, due 2019
Secured Long-term debt:
Interest rate (as a percent)	6.10%	6.10%
Public | First mortgage bonds payable: Series 2009B First Mortgage Bonds, 5.95%, due 2039
Secured Long-term debt:
Interest rate (as a percent)	5.95%	5.95%
Public | First mortgage bonds payable: Series 2009 Clean renewable energy bond, 1.81%, due 2024
Secured Long-term debt:
Interest rate (as a percent)	1.81%	1.81%
Public | First mortgage bonds payable: Series 2010A First Mortgage Bonds, 5.375% due 2040
Secured Long-term debt:
Interest rate (as a percent)	5.375%	5.375%
Public | First mortgage bonds payable: Series 2011A First Mortgage Bonds, 5.25% due 2050
Secured Long-term debt:
Interest rate (as a percent)	5.25%	5.25%
Public | First mortgage bonds payable: Series 2012A First Mortgage Bonds, 4.20% due 2042
Secured Long-term debt:
Interest rate (as a percent)	4.20%	4.20%
Public | First mortgage bonds payable: Series 2014A First Mortgage Bonds, 4.55% due 2044
Secured Long-term debt:
Interest rate (as a percent)	4.55%	4.55%
Public | First Mortgage Bonds Payable: Series 2016 First Mortgage Bonds, 4.25% due 2046
Secured Long-term debt:
Interest rate (as a percent)	4.25%
Georgia Development Authorities | First mortgage notes issued in connection with the sale of pollution control revenue bonds: Series 2003A Burke, Heard, Monroe and 2003B Burke Auction rate bonds, 1.24%, due 2024
Secured Long-term debt:
Interest rate (as a percent)	1.24%	1.24%
Georgia Development Authorities | First mortgage notes issued in connection with the sale of pollution control revenue bonds: Series 2004 Burke and Monroe Auction rate bonds, 1.35%, due 2020
Secured Long-term debt:
Interest rate (as a percent)	1.35%	1.35%
Georgia Development Authorities | First mortgage notes issued in connection with the sale of pollution control revenue bonds: Series 2005 Burke and Monroe Auction rate bonds, 1.06%, due 2040
Secured Long-term debt:
Interest rate (as a percent)	1.06%	1.06%
Georgia Development Authorities | First mortgage notes issued in connection with the sale of pollution control revenue bonds: Series 2008A through 2008C Burke Fixed rate bonds, 5.30% to 5.70%, due 2032 through 2043
Secured Long-term debt:
Interest rate, minimum (as a percent)	5.30%	5.30%
Interest rate, maximum (as a percent)	5.70%	5.70%
Georgia Development Authorities | First mortgage notes issued in connection with the sale of pollution control revenue bonds: Series 2008E Burke Fixed rate bonds, 7.00%, due 2020 through 2023
Secured Long-term debt:
Interest rate (as a percent)	7.00%	7.00%
Georgia Development Authorities | First mortgage notes issued in connection with the sale of pollution control revenue bonds: Series 2009A Heard and Monroe, and 2009B Monroe Weekly rate bonds, 0.73% to 0.76%, due 2030 through 2038
Secured Long-term debt:
Interest rate, minimum (as a percent)	0.73%	0.73%
Interest rate, maximum (as a percent)	0.76%	0.76%
Georgia Development Authorities | First mortgage notes issued in connection with the sale of pollution control revenue bonds: 2010A Burke and Monroe, and 2010B Burke Weekly rate bonds, 0.72% to 0.73%, due 2036 through 2037
Secured Long-term debt:
Interest rate, minimum (as a percent)	0.72%	0.72%
Interest rate, maximum (as a percent)	0.73%	0.73%
Georgia Development Authorities | First mortgage notes issued in connection with the sale of pollution control revenue bonds: 2013A Appling, Burke and Monroe Term rate bonds, 2.40% through April 1, 2020, due 2038 through 2040
Secured Long-term debt:
Interest rate (as a percent)	2.40%	2.40%
CoBank | ACB notes payable: Transmission first mortgage notes payable: variable at 2.65% to 3.75% through January 31, 2017, due in bimonthly installments through November 1, 2018
Secured Long-term debt:
Interest rate, minimum (as a percent)	2.65%	2.65%
Interest rate, maximum (as a percent)	3.75%	3.75%
CoBank | ACB notes payable: Transmission first mortgage notes payable: variable at 2.65% to 3.75% through January 31, 2017, due in bimonthly installments through September 1, 2019
Secured Long-term debt:
Interest rate, minimum (as a percent)	2.65%	2.65%
Interest rate, maximum (as a percent)	3.75%	3.75%