Contingencies and Regulatory Matters (Details)				3 Months Ended	12 Months Ended
	Aug. 20, 2014 item	Jul. 28, 2014 item	Mar. 13, 2014 item	Mar. 31, 2016 item	Dec. 31, 2008 item MW
Patronage Capital Litigation
Number of electric distribution cooperative members				38
Vogtle Units Number 3 And Number 4
Nuclear Construction
Number of Westinghouse AP1000 nuclear generating units					2
Nominally rated generating capacity for each unit | MW					1,100
Ownership interest of nuclear units (as a percent)					30.00%
Nominally rated generating capacity | MW					660
Patronage Capital Litigation
Patronage Capital Litigation
Number of defendant members	2		3
Number of excluded members against whom defendant class action is certified		1
Number of electric distribution cooperative members		38
Number of former consumer members		4
Number of members in which the former consumer members filing the lawsuit belonged		4
Percentage of cooperatives total booked patronage capital		30.00%
Number of current consumer members	2
Number of plaintiffs	3
Patronage Capital Litigation | Maximum
Patronage Capital Litigation
Period of revolving schedule to retire the patronage capital of former consumer-members		13 years
Period of revolving schedule to adopt policies to periodically retire the patronage capital of all consumer-members		13 years