Income Taxes - Impact of Unrecognized Tax Benefits on Effective Tax Rate, If Recognized (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Impact on effective tax rate
Tax positions impacting the effective tax rate	$ 10	$ 10	$ 7
Tax positions not impacting the effective tax rate	423	160	0
Balance of unrecognized tax benefits	433	170	7	$ 70
Mississippi Power [Member]
Impact on effective tax rate
Tax positions impacting the effective tax rate	(2)	4	4
Tax positions not impacting the effective tax rate	423	161	0
Balance of unrecognized tax benefits	$ 421	$ 165	$ 4	$ 6

Amount of unrecognized tax benefits pertaining to uncertain tax positions taken in tax returns.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 15A
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6907707&loc=SL6600010-109319

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Unrecognized Tax Benefit
-URI http://asc.fasb.org/extlink&oid=6527854

Name:	us-gaap_UnrecognizedTaxBenefits
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant