Proposed Merger with Southern Company (Details) - USD ($)		3 Months Ended		9 Months Ended
	Aug. 23, 2015	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Business Acquisition [Line Items]
Merger-related expenses		$ 35,000,000	$ 0	$ 35,000,000	$ 0
Southern Company [Member]
Business Acquisition [Line Items]
Common stock right to convert to cash, price per share (in dollars per share)	$ 66
Merger termination fee	$ 201,000,000
Reimbursement of expenses upon termination, up to	$ 5,000,000
Share-based compensation expense				19,000,000
Financial advisory, legal and other merger expense				13,000,000
Operation and Maintenance Expense [Member] | Southern Company [Member]
Business Acquisition [Line Items]
Merger-related expenses		35,000,000		35,000,000
Merger related costs, net of tax		$ 21,000,000		21,000,000
Board of Directors [Member] | Southern Company [Member]
Business Acquisition [Line Items]
Board of directors stock-based compensation				$ 3,000,000