Summary of significant accounting policies: - Regulatory assets and liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 545,387	$ 530,254
Total Regulatory Liabilities	197,748	166,967
Net Regulatory Assets	347,639	363,287
Accumulated retirement costs for other obligations
Regulatory Assets and Liabilities
Total Regulatory Liabilities	9,829	8,910
Deferral of effects on net margin- Hawk Road Energy Facility | Hawk Road
Regulatory Assets and Liabilities
Total Regulatory Liabilities	20,163	20,775
Major maintenance reserve
Regulatory Assets and Liabilities
Total Regulatory Liabilities	28,379	22,422
Amortization on capital leases
Regulatory Assets and Liabilities
Total Regulatory Liabilities	23,084	26,502
Deferred debt service adder
Regulatory Assets and Liabilities
Total Regulatory Liabilities	86,082	76,334
Asset retirement obligations | Nuclear
Regulatory Assets and Liabilities
Total Regulatory Liabilities	11,766	8,316
Other regulatory liabilities
Regulatory Assets and Liabilities
Total Regulatory Liabilities	$ 18,445	3,708
Other regulatory liabilities | Maximum
Regulatory Assets and Liabilities
Amortization period, other regulatory liabilities	10 years
Premium and loss on reacquired debt
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 55,084	61,916
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	$ 32,274	30,253
Outage costs
Regulatory Assets and Liabilities
Total Regulatory Assets	39,986	42,027
Interest rate swap termination fees
Regulatory Assets and Liabilities
Total Regulatory Assets	3,570	5,355
Asset retirement obligations | Ashpond and Other Decommissioning
Regulatory Assets and Liabilities
Total Regulatory Assets	33,747	1,098
Depreciation expense
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 44,091	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	$ 43,444	37,646
Interest rate options cost
Regulatory Assets and Liabilities
Total Regulatory Assets	107,394	102,554
Deferral of effects on net margin - Smith Energy Facility | Smith
Regulatory Assets and Liabilities
Total Regulatory Assets	172,399	178,343
Other regulatory assets
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 13,398	$ 25,548
Other regulatory assets | Maximum
Regulatory Assets and Liabilities
Amortization period, other regulatory assets	33 years
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