Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 609,919	$ 585,084
Total Regulatory Liabilities	272,427	251,649
Net Regulatory Assets	337,492	333,435
Accumulated retirement costs for other obligations
Regulatory Assets and Liabilities
Total Regulatory Liabilities	16,499	12,813
Deferral of effects on net margin | Hawk Road Energy Facility
Regulatory Assets and Liabilities
Total Regulatory Liabilities	19,094	19,553
Major maintenance reserve
Regulatory Assets and Liabilities
Total Regulatory Liabilities	53,587	47,087
Amortization on capital leases
Regulatory Assets and Liabilities
Total Regulatory Liabilities	17,880	20,055
Deferred debt service adder
Regulatory Assets and Liabilities
Total Regulatory Liabilities	102,819	95,695
Asset retirement obligations
Regulatory Assets and Liabilities
Total Regulatory Liabilities	49,210	53,571
Other regulatory liabilities
Regulatory Assets and Liabilities
Total Regulatory Liabilities	$ 13,338	2,875
Other regulatory liabilities | Maximum
Regulatory Assets and Liabilities
Amortization Period	9 years
Premium and loss on reacquired debt
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 47,981	52,989
Premium and loss on reacquired debt | Maximum
Regulatory Assets and Liabilities
Amortization period, other regulatory assets	26 years
Amortization on capital leases
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 34,650	33,846
Outage costs
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 37,080	40,525
Coal-fired maintenance outage costs | Maximum
Regulatory Assets and Liabilities
Amortization period, other regulatory assets	48 months
Nuclear refueling outage costs | Minimum
Regulatory Assets and Liabilities
Amortization period, other regulatory assets	18 months
Nuclear refueling outage costs | Maximum
Regulatory Assets and Liabilities
Amortization period, other regulatory assets	24 months
Asset retirement obligations | Ashpond and other
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 93,842	68,289
Depreciation expense
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 41,600	42,667
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	$ 50,888	48,702
Interest rate options cost
Regulatory Assets and Liabilities
Total Regulatory Assets	115,734	112,102
Deferral of effects on net margin | Smith Energy Facility
Regulatory Assets and Liabilities
Total Regulatory Assets	161,995	166,454
Other regulatory assets
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 26,149	$ 19,510
Other regulatory assets | Maximum
Regulatory Assets and Liabilities
Amortization period, other regulatory assets	32 years