Contingencies and Regulatory Matters - Other Matters (Details) $ in Millions	3 Months Ended
Mar. 31, 2018 USD ($)
Southern Company Holdings Subsidiary
Loss Contingencies [Line Items]
Leveraged leases, balance sheet, investment in leveraged leases	$ 86
Natural Gas Storage
Loss Contingencies [Line Items]
Public utilities, property, plant and equipment, net	$ 111
Natural Gas Storage - Salt Dome Caverns
Loss Contingencies [Line Items]
Public utilities, property, plant and equipment, percentage of total	20.00%

The net investment in arrangements meeting the criteria to be classified as leveraged excluding deferred taxes.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 30
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=77907235&loc=d3e45152-112736

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 50
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82918944&loc=SL77920254-209983

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 30
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=84165051&loc=d3e45424-112738

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant