Joint Ownership Agreements - Narrative (Details) - SNG - SOUTHERN Co GAS - USD ($) $ in Millions	Mar. 31, 2017	Sep. 30, 2016
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		50.00%
Capital contributions from parent company	$ 50

The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=66011266&loc=d3e33918-111571

Name:	us-gaap_EquityMethodInvestmentOwnershipPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant