Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Regulatory assets and liabilities
Total Regulatory Assets	$ 513,026	$ 484,049
Total Regulatory Liabilities	192,970	194,073
Net Regulatory Assets	320,056	289,976
Accumulated retirement costs for other obligations
Regulatory assets and liabilities
Total Regulatory Liabilities	14,332	18,559
Deferral of effects on net margin- Hawk Road Energy Facility | Hawk Road
Regulatory assets and liabilities
Total Regulatory Liabilities	28,622	29,867
Major maintenance reserve
Regulatory assets and liabilities
Total Regulatory Liabilities	19,133	23,427
Amortization on capital leases
Regulatory assets and liabilities
Total Regulatory Liabilities	26,198	21,693
Deferred debt service adder
Regulatory assets and liabilities
Total Regulatory Liabilities	71,527	66,754
Asset retirement obligations
Regulatory assets and liabilities
Total Regulatory Liabilities	28,277	28,870
Other regulatory liabilities
Regulatory assets and liabilities
Total Regulatory Liabilities	$ 4,881	4,903
Other regulatory liabilities | Maximum
Regulatory assets and liabilities
Amortization period	18 years
Premium and loss on reacquired debt
Regulatory assets and liabilities
Total Regulatory Assets	$ 66,823	71,731
Premium and loss on reacquired debt | Maximum
Regulatory assets and liabilities
Amortization period	30 years
Amortization on capital leases
Regulatory assets and liabilities
Total Regulatory Assets	$ 29,041	27,829
Outage costs
Regulatory assets and liabilities
Total Regulatory Assets	41,616	45,795
Interest rate swap termination fees
Regulatory assets and liabilities
Total Regulatory Assets	7,350	9,345
Depreciation expense
Regulatory assets and liabilities
Total Regulatory Assets	$ 46,226	46,938
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 | Vogtle Units Number 3 And Number 4
Regulatory assets and liabilities
Total Regulatory Assets	$ 35,083	32,501
Interest rate options cost
Regulatory assets and liabilities
Total Regulatory Assets	98,577	98,671
Deferral of effects on net margin - Smith Energy Facility | Smith
Regulatory assets and liabilities
Total Regulatory Assets	169,188	128,666
Other regulatory assets
Regulatory assets and liabilities
Total Regulatory Assets	$ 19,122	$ 22,573
Other regulatory assets | Maximum
Regulatory assets and liabilities
Amortization period	35 years
Coal-fired outage costs
Regulatory assets and liabilities
Amortization period	24 months
Nuclear refueling outage costs | Minimum
Regulatory assets and liabilities
Amortization period	18 months
Nuclear refueling outage costs | Maximum
Regulatory assets and liabilities
Amortization period	24 months