Summary of significant accounting policies: - Estimated costs of nuclear decommissioning (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($)
Nuclear decommissioning
Assumed escalation rate for labor, material and equipment (as a percent)	2.50%
Hatch Unit No. 1
Nuclear decommissioning
Estimated costs based on site study	$ 258,000
Hatch Unit No. 2
Nuclear decommissioning
Estimated costs based on site study	282,000
Vogtle Unit No. 1
Nuclear decommissioning
Estimated costs based on site study	253,000
Vogtle Unit No. 2
Nuclear decommissioning
Estimated costs based on site study	275,000
Radiated structures | Hatch Unit No. 1
Nuclear decommissioning
Estimated costs based on site study	193,000
Radiated structures | Hatch Unit No. 2
Nuclear decommissioning
Estimated costs based on site study	213,000
Radiated structures | Vogtle Unit No. 1
Nuclear decommissioning
Estimated costs based on site study	178,000
Radiated structures | Vogtle Unit No. 2
Nuclear decommissioning
Estimated costs based on site study	195,000
Spent fuel management | Hatch Unit No. 1
Nuclear decommissioning
Estimated costs based on site study	49,000
Spent fuel management | Hatch Unit No. 2
Nuclear decommissioning
Estimated costs based on site study	47,000
Spent fuel management | Vogtle Unit No. 1
Nuclear decommissioning
Estimated costs based on site study	49,000
Spent fuel management | Vogtle Unit No. 2
Nuclear decommissioning
Estimated costs based on site study	47,000
Non-radiated structures | Hatch Unit No. 1
Nuclear decommissioning
Estimated costs based on site study	16,000
Non-radiated structures | Hatch Unit No. 2
Nuclear decommissioning
Estimated costs based on site study	22,000
Non-radiated structures | Vogtle Unit No. 1
Nuclear decommissioning
Estimated costs based on site study	26,000
Non-radiated structures | Vogtle Unit No. 2
Nuclear decommissioning
Estimated costs based on site study	$ 33,000