Regulatory Assets and Liabilities (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2012	Dec. 31, 2011
Regulatory assets and liabilities
Total Regulatory Assets	$ 357,600	$ 351,547
Total Regulatory Liabilities	133,383	164,000
Net Regulatory Assets	224,217	187,547
Accumulated retirement costs for other obligations
Regulatory assets and liabilities
Total Regulatory Liabilities	30,071	32,687
Net benefit of Rocky Mountain transactions | Rocky Mountain
Regulatory assets and liabilities
Total Regulatory Liabilities	14,056	47,783
Amortization period	30 years
Deferral of effects on net margin - Hawk Road Energy Facility | Hawk Road and Murray Energy
Regulatory assets and liabilities
Total Regulatory Liabilities	12,773	15,811
Major maintenance sinking fund
Regulatory assets and liabilities
Total Regulatory Liabilities	30,101	28,524
Deferred debt service adder
Regulatory assets and liabilities
Total Regulatory Liabilities	45,012	37,586
Other regulatory liabilities
Regulatory assets and liabilities
Total Regulatory Liabilities	1,370	1,609
Other regulatory liabilities | Maximum
Regulatory assets and liabilities
Amortization period	7 years
Premium and loss on reacquired debt
Regulatory assets and liabilities
Total Regulatory Assets	89,373	98,538
Premium and loss on reacquired debt | Maximum
Regulatory assets and liabilities
Amortization period	31 years
Amortization on capital leases
Regulatory assets and liabilities
Total Regulatory Assets	33,055	46,627
Outage costs
Regulatory assets and liabilities
Total Regulatory Assets	36,536	42,866
Outage costs | Minimum
Regulatory assets and liabilities
Amortization period	18 months
Outage costs | Maximum
Regulatory assets and liabilities
Amortization period	36 months
Interest rate swap termination fees
Regulatory assets and liabilities
Total Regulatory Assets	18,324	21,316
Asset retirement obligations
Regulatory assets and liabilities
Total Regulatory Assets	10,990	29,341
Depreciation expense
Regulatory assets and liabilities
Total Regulatory Assets	50,141	51,209
Depreciation expense | Plant Vogtle
Regulatory assets and liabilities
Operating license expected extension period for Plant Vogtle	20 years
Operating license, expected extension period, for Plant Vogtle	40 years
Deferred charges related to Vogtle Units No. 3 and No. 4 training costs | Plant Vogtle
Regulatory assets and liabilities
Total Regulatory Assets	23,336	17,602
Interest rate options cost
Regulatory assets and liabilities
Total Regulatory Assets	71,324	30,735
Deferral of effects on net margin - Smith Energy Facility
Regulatory assets and liabilities
Total Regulatory Assets	15,936	3,536
Other regulatory assets
Regulatory assets and liabilities
Total Regulatory Assets	$ 8,585	$ 9,777
Other regulatory assets | Maximum
Regulatory assets and liabilities
Amortization period	37 years