Summary of significant accounting policies: - Nuclear fuel cost information (Details) - USD ($)	1 Months Ended	12 Months Ended
	Apr. 30, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 14, 2014
Nuclear fuel cost
Nuclear fuel expense		$ 83,751,000	$ 78,762,000	$ 85,166,000
Nuclear Fuel Disposal Cost Settlement, January 1, 2005 through December 31, 2010 | Plant Hatch and Plant Vogtle
Nuclear fuel cost
Settlement amount, all parties					$ 36,474,000
Settlement amount, entity share	$ 10,949,000
Reduction of total operating expenses	7,320,000
Reduction to plant in service	$ 3,629,000
Nuclear Fuel Disposal Cost Settlement, January 1, 2011 through December 31, 2013 | Plant Hatch and Plant Vogtle
Nuclear fuel cost
Damages receivable		$ 0