Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 773,780	$ 655,063
Total Regulatory Liabilities	299,527	218,998
Net Regulatory Assets	474,253	436,065
Accumulated retirement costs for other obligations
Regulatory Assets and Liabilities
Total Regulatory Liabilities	14,092	13,873
Deferral of effects on net margin | Hawk Road Energy Facility
Regulatory Assets and Liabilities
Total Regulatory Liabilities	18,639	19,101
Major maintenance reserve
Regulatory Assets and Liabilities
Total Regulatory Liabilities	46,907	45,547
Amortization of financing leases
Regulatory Assets and Liabilities
Total Regulatory Liabilities	14,981	17,156
Deferred debt service adder
Regulatory Assets and Liabilities
Total Regulatory Liabilities	112,144	105,192
Asset Retirement Obligations
Regulatory Assets and Liabilities
Total Regulatory Liabilities	37,879
Revenue deferral plan
Regulatory Assets and Liabilities
Total Regulatory Liabilities	52,003	15,670
Other regulatory liabilities
Regulatory Assets and Liabilities
Total Regulatory Liabilities	$ 2,882	2,459
Other regulatory liabilities | Maximum
Regulatory Assets and Liabilities
Amortization Period	8 years
Premium and loss on reacquired debt
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 41,580	46,315
Premium and loss on reacquired debt | Maximum
Regulatory Assets and Liabilities
Amortization period, other regulatory assets	25 years
Amortization of financing leases
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 35,304	34,918
Outage costs
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 36,272	36,352
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	$ 258,227	137,835
Asset Retirement Obligations | Nuclear
Regulatory Assets and Liabilities
Total Regulatory Assets		7,031
Depreciation expense
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 40,176	41,244
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 No. 3 & No. 4
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 52,802	51,549
Interest rate options cost
Regulatory Assets and Liabilities
Total Regulatory Assets	120,685	116,960
Deferral of effects on net margin - Smith Energy Facility | Smith Energy Facility
Regulatory Assets and Liabilities
Total Regulatory Assets	156,051	160,509
Other regulatory assets
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 32,683	$ 22,350
Other regulatory assets | Maximum
Regulatory Assets and Liabilities
Amortization period, other regulatory assets	31 years