Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 554,494	$ 530,254
Total Regulatory Liabilities	177,351	166,967
Net Regulatory Assets	377,143	363,287
Accumulated retirement costs for other obligations
Regulatory Assets and Liabilities
Total Regulatory Liabilities	10,728	8,910
Deferral of effects on net margin- Hawk Road Energy Facility | Hawk Road
Regulatory Assets and Liabilities
Total Regulatory Liabilities	20,622	20,775
Major maintenance reserve
Regulatory Assets and Liabilities
Total Regulatory Liabilities	24,872	22,422
Amortization on capital leases
Regulatory Assets and Liabilities
Total Regulatory Liabilities	25,647	26,502
Deferred debt service adder
Regulatory Assets and Liabilities
Total Regulatory Liabilities	78,767	76,334
Asset retirement obligations.
Regulatory Assets and Liabilities
Total Regulatory Liabilities	12,917	8,316
Other regulatory liabilities
Regulatory Assets and Liabilities
Total Regulatory Liabilities	$ 3,798	3,708
Other regulatory liabilities | Maximum
Regulatory Assets and Liabilities
Amortization Period	11 years
Premium and loss on reacquired debt
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 60,147	61,916
Premium and loss on reacquired debt | Maximum
Regulatory Assets and Liabilities
Amortization period	28 years
Amortization on capital leases
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 30,759	30,253
Outage costs
Regulatory Assets and Liabilities
Total Regulatory Assets	56,327	42,027
Interest rate swap termination fees
Regulatory Assets and Liabilities
Total Regulatory Assets	4,909	5,355
Depreciation expense
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 45,159	45,514
Depreciation expense | Plant Vogtle
Regulatory Assets and Liabilities
Operating license expected extension period for Plant Vogtle	20 years
Operating license period	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	$ 38,899	37,646
Interest rate options cost
Regulatory Assets and Liabilities
Total Regulatory Assets	103,713	102,554
Deferral of effects on net margin - Smith Energy Facility | Smith
Regulatory Assets and Liabilities
Total Regulatory Assets	176,857	178,343
Other regulatory assets
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 37,724	$ 26,646
Other regulatory assets | Maximum
Regulatory Assets and Liabilities
Amortization period	34 years
Coal-fired maintenance 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