Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 542,057	$ 530,254
Total Regulatory Liabilities	193,156	166,967
Net Regulatory Assets	348,901	363,287
Accumulated retirement costs for other obligations
Regulatory Assets and Liabilities
Total Regulatory Liabilities	12,925	8,910
Deferral of effects on net margin- Hawk Road Energy Facility | Hawk Road
Regulatory Assets and Liabilities
Total Regulatory Liabilities	20,316	20,775
Major maintenance reserve
Regulatory Assets and Liabilities
Total Regulatory Liabilities	29,450	22,422
Amortization on capital leases
Regulatory Assets and Liabilities
Total Regulatory Liabilities	23,939	26,502
Deferred debt service adder
Regulatory Assets and Liabilities
Total Regulatory Liabilities	83,644	76,334
Asset retirement obligations.
Regulatory Assets and Liabilities
Total Regulatory Liabilities	19,106	8,316
Other regulatory liabilities
Regulatory Assets and Liabilities
Total Regulatory Liabilities	$ 3,776	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	$ 56,772	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	$ 31,769	30,253
Outage costs
Regulatory Assets and Liabilities
Total Regulatory Assets	39,589	42,027
Interest rate swap termination fees
Regulatory Assets and Liabilities
Total Regulatory Assets	4,017	5,355
Depreciation expense
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 44,447	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	$ 41,871	37,646
Interest rate options cost
Regulatory Assets and Liabilities
Total Regulatory Assets	106,220	102,554
Deferral of effects on net margin - Smith Energy Facility | Smith
Regulatory Assets and Liabilities
Total Regulatory Assets	173,885	178,343
Other regulatory assets
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 43,487	$ 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