Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 572,237	$ 545,387
Total Regulatory Liabilities	236,445	197,748
Net Regulatory Assets	335,792	347,639
Accumulated retirement costs for other obligations
Regulatory Assets and Liabilities
Total Regulatory Liabilities	14,235	9,829
Deferral of effects on net margin- Hawk Road Energy Facility | Hawk Road
Regulatory Assets and Liabilities
Total Regulatory Liabilities	19,705	20,163
Major maintenance reserve
Regulatory Assets and Liabilities
Total Regulatory Liabilities	43,269	28,379
Amortization on capital leases
Regulatory Assets and Liabilities
Total Regulatory Liabilities	20,780	23,084
Deferred debt service adder
Regulatory Assets and Liabilities
Total Regulatory Liabilities	93,296	86,082
Asset retirement obligations
Regulatory Assets and Liabilities
Total Regulatory Liabilities	40,199	11,766
Other regulatory liabilities
Regulatory Assets and Liabilities
Total Regulatory Liabilities	$ 4,961	18,445
Other regulatory liabilities | Maximum
Regulatory Assets and Liabilities
Amortization Period	10 years
Premium and loss on reacquired debt
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 51,546	55,084
Premium and loss on reacquired debt | Maximum
Regulatory Assets and Liabilities
Amortization period, other regulatory assets	27 years
Amortization on capital leases
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 33,454	32,274
Outage costs
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 42,060	39,986
Coal-fired maintenance outage costs
Regulatory Assets and Liabilities
Amortization period, other regulatory assets	24 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
Interest rate swap termination fees
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 2,231	3,570
Asset retirement obligations
Regulatory Assets and Liabilities
Total Regulatory Assets	59,540	33,747
Depreciation expense
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 43,023	44,091
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	$ 47,322	43,444
Interest rate options cost
Regulatory Assets and Liabilities
Total Regulatory Assets	110,915	107,394
Deferral of effects on net margin - Smith Energy Facility | Smith
Regulatory Assets and Liabilities
Total Regulatory Assets	167,941	172,399
Other regulatory assets
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 14,205	$ 13,398
Other regulatory assets | Maximum
Regulatory Assets and Liabilities
Amortization period, other regulatory assets	33 years