Regulatory Assets and Liabilities (Details) (USD $) In Thousands, unless otherwise specified											12 Months Ended			12 Months Ended										12 Months Ended											12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2013 Accumulated retirement costs for other obligations	Dec. 31, 2012 Accumulated retirement costs for other obligations	Mar. 31, 2013 Major maintenance sinking fund	Dec. 31, 2012 Major maintenance sinking fund	Mar. 31, 2013 Deferred debt service adder	Dec. 31, 2012 Deferred debt service adder	Mar. 31, 2013 Other regulatory liabilities	Dec. 31, 2012 Other regulatory liabilities	Dec. 31, 2012 Other regulatory liabilities Maximum	Mar. 31, 2013 Premium and loss on required debt	Dec. 31, 2012 Premium and loss on required debt	Dec. 31, 2012 Premium and loss on required debt Maximum	Mar. 31, 2013 Amortization on capital leases	Dec. 31, 2012 Amortization on capital leases	Mar. 31, 2013 Outage costs	Dec. 31, 2012 Outage costs	Mar. 31, 2013 Interest rate swap termination fees	Dec. 31, 2012 Interest rate swap termination fees	Dec. 31, 2012 Asset retirement obligations	Mar. 31, 2013 Depreciation expense	Dec. 31, 2012 Depreciation expense	Dec. 31, 2012 Depreciation expense Plant Vogtle	Mar. 31, 2013 Deferred charges related to Vogtle Units No. 3 and No. 4 training costs Vogtle Units No.3 and No.4	Dec. 31, 2012 Deferred charges related to Vogtle Units No. 3 and No. 4 training costs Vogtle Units No.3 and No.4	Mar. 31, 2013 Interest rate options cost	Dec. 31, 2012 Interest rate options cost	Mar. 31, 2013 Deferral of effects on net margin - Smith Energy Facility Smith	Dec. 31, 2012 Deferral of effects on net margin - Smith Energy Facility Smith	Mar. 31, 2013 Deferral of effects on net margin - Smith Energy Facility Hawk Road	Dec. 31, 2012 Deferral of effects on net margin - Smith Energy Facility Hawk Road	Mar. 31, 2013 Other regulatory assets	Dec. 31, 2012 Other regulatory assets	Dec. 31, 2012 Other regulatory assets Maximum	Dec. 31, 2012 Coal-fired outage costs Minimum	Dec. 31, 2012 Coal-fired outage costs Maximum	Dec. 31, 2012 Nuclear refueling outage costs Minimum	Dec. 31, 2012 Nuclear refueling outage costs Maximum
Regulatory assets and liabilities
Total Regulatory Assets	$ 361,614	$ 352,902										$ 83,292	$ 86,319		$ 25,536	$ 28,670	$ 45,294	$ 30,901	$ 16,328	$ 17,326	$ 11,382	$ 49,429	$ 49,785		$ 24,423	$ 23,030	$ 75,192	$ 75,716	$ 35,279	$ 21,394			$ 6,841	$ 8,379
Total Regulatory Liabilities	133,401	129,985	27,529	28,846	29,861	30,948	49,911	47,486	7,005	5,592																					19,095	17,113
Net Regulatory Assets	$ 228,213	$ 222,917
Amortization period														30 years																					36 years	18 months	36 months
Operating license expected extension period for Plant Vogtle																								20 years
Operating license, expected extension period, for Plant Vogtle																								40 years
Amortization period											13 years																											18 months	24 months