Proposed Merger with Southern Company (Details) - USD ($)		12 Months Ended
	Aug. 23, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Business Acquisition [Line Items]
Merger-related expenses		$ 44,000,000	$ 0	$ 0
Share-based compensation expense		40,000,000	$ 24,000,000	$ 22,000,000
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
Merger termination fee liability		0
Share-based compensation expense		24,000,000
Financial advisory, legal and other merger expense		16,000,000
Southern Company [Member] | Operation and Maintenance Expense [Member]
Business Acquisition [Line Items]
Merger-related expenses		44,000,000
Merger related expenses, net of tax		26,000,000
Director [Member] | Southern Company [Member]
Business Acquisition [Line Items]
Board of directors stock-based compensation		$ 4,000,000