Electric plant, construction and related agreements: Construction information (Details) - Vogtle Units Number 3 And Number 4				12 Months Ended
	Jan. 01, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 30, 2015 USD ($)	Dec. 31, 2008 USD ($) item MW	Jan. 13, 2016 USD ($)	Jun. 30, 2015 USD ($)
Construction
Number of Westinghouse AP1000 nuclear generating units | item				2
Nominally rated generating capacity for each unit | MW				1,100
Ownership interest of nuclear units (as a percent)				30.00%
Amount to be paid to the Contractor and capitalized to the project, contingent upon acquisition		$ 230,000,000
Payments for Legal Settlements	$ 80,000,000		$ 80,000,000
Project budget including capital costs, allowance for funds used during construction and contingency amount		5,000,000,000
Total investment in additional vogtle units		$ 2,888,000,000
Aggregate Damages Under Initial Complaint And Amended Counterclaim
Construction
Maximum additional capital cost under the provision						$ 470,000,000
Amended counterclaim associated with design changes, delays in Project Schedule and issuance of permits and operating licenses
Construction
Maximum additional capital cost under the provision				$ 75,000,000
Toshiba Corporation | Letter of credit
Construction
Letters of credit					$ 900,000,000