Contingencies and Regulatory Matters - Textual - SO Other Matters (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Schedule of Regulatory Assets and Liabilities [Line Items]
Property, plant and equipment	$ 79,872,000,000	$ 78,446,000,000
Natural Gas Storage - Salt Dome Caverns
Schedule of Regulatory Assets and Liabilities [Line Items]
Property, plant and equipment	$ 112,000,000
Property, Plant and Equipment | Natural Gas Storage - Salt Dome Caverns
Schedule of Regulatory Assets and Liabilities [Line Items]
Concentration risk (as percent)	20.00%
SOUTHERN Co GAS
Schedule of Regulatory Assets and Liabilities [Line Items]
Property, plant and equipment	$ 11,728,000,000	$ 10,565,000,000
Impairment of long-lived assets	$ 0