Summary of significant accounting policies - Regulatory assets and liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 763,512	$ 655,063
Total Regulatory Liabilities	364,241	218,998
Net Regulatory Assets	399,271	436,065
Accumulated retirement costs for other obligations
Regulatory Assets and Liabilities
Total Regulatory Liabilities	12,692	13,873
Deferral of effects on net margin | Hawk Road Energy Facility
Regulatory Assets and Liabilities
Total Regulatory Liabilities	18,485	19,101
Major maintenance reserve
Regulatory Assets and Liabilities
Total Regulatory Liabilities	50,144	45,547
Amortization on capital leases
Regulatory Assets and Liabilities
Total Regulatory Liabilities	14,256	17,156
Deferred debt service adder
Regulatory Assets and Liabilities
Total Regulatory Liabilities	114,453	105,192
Nuclear
Regulatory Assets and Liabilities
Total Regulatory Liabilities	61,516	0
Revenue deferral plan
Regulatory Assets and Liabilities
Total Regulatory Liabilities	$ 90,066	15,670
Amortization period, other regulatory liabilities	5 years
Other regulatory liabilities
Regulatory Assets and Liabilities
Total Regulatory Liabilities	$ 2,629	2,459
Other regulatory liabilities | Maximum
Regulatory Assets and Liabilities
Amortization period, other regulatory liabilities	7 years
Premium and loss on reacquired debt
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 40,067	46,315
Premium and loss on reacquired debt | Maximum
Regulatory Assets and Liabilities
Amortization period, other regulatory assets	24 years
Amortization on financing leases
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 35,433	34,918
Outage costs
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 34,367	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	$ 245,932	137,835
Asset retirement obligations - Nuclear
Regulatory Assets and Liabilities
Total Regulatory Assets	0	7,031
Depreciation expense
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 39,820	41,244
Depreciation expense | Plant Vogtle
Regulatory Assets and Liabilities
Operating license period	40 years
Operating license expected extension period for Plant Vogtle	20 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	$ 53,466	51,549
Interest rate options cost
Regulatory Assets and Liabilities
Total Regulatory Assets	121,938	116,960
Deferral of effects on net margin | Smith Energy Facility
Regulatory Assets and Liabilities
Total Regulatory Assets	154,564	160,509
Other regulatory assets
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 37,925	$ 22,350
Other regulatory assets | Maximum
Regulatory Assets and Liabilities
Amortization period, other regulatory assets	30 years