Contingencies and Regulatory Matters - Textual - SO Regulatory Matters GAS (Details) - USD ($)							1 Months Ended	12 Months Ended
	Jan. 31, 2018	Jan. 29, 2018	Jan. 01, 2018	Dec. 21, 2017	Jun. 30, 2017	Feb. 21, 2017	Aug. 31, 2016	Dec. 31, 2017	Jun. 01, 2018	Jan. 11, 2018
SOUTHERN Co GAS
Schedule of Regulatory Assets and Liabilities [Line Items]
Impairment of long-lived assets								$ 0
Atlanta Gas Light
Schedule of Regulatory Assets and Liabilities [Line Items]
Public utilities, approved rate increase (decrease), amount				$ 34,000,000	$ 13,000,000	$ 20,000,000
Public utilities, approved rate increase (decrease) amount recovery of investments				$ 13,000,000
Public utilities, approved return on equity				9.50%	9.60%	10.75%
Atlanta Gas Light | Minimum
Schedule of Regulatory Assets and Liabilities [Line Items]
Public utilities, approved return on equity				9.00%		10.55%
Atlanta Gas Light | Maximum
Schedule of Regulatory Assets and Liabilities [Line Items]
Public utilities, approved return on equity				10.00%		10.95%
Infrastructure Program | SOUTHERN Co GAS | Minimum
Schedule of Regulatory Assets and Liabilities [Line Items]
Program duration period								9 years
Infrastructure Program | SOUTHERN Co GAS | Maximum
Schedule of Regulatory Assets and Liabilities [Line Items]
Program duration period								10 years
energySMART [Member] | Nicor Gas
Schedule of Regulatory Assets and Liabilities [Line Items]
Program duration period						4 years
Subsequent Event | Atlanta Gas Light
Schedule of Regulatory Assets and Liabilities [Line Items]
Public utilities, approved rate increase (decrease), amount	$ 137,000,000
Public utilities, approved rate increase (decrease) amount recovery of investments	$ 93,000,000
Public utilities, approved return on equity	9.80%
Subsequent Event | Florida City Gas
Schedule of Regulatory Assets and Liabilities [Line Items]
Tax Cuts and Jobs Acts of 2017, loss contingency, Damages Sought, reduction in base rate revenues, value		$ 4,000,000
Subsequent Event | energySMART [Member] | Nicor Gas
Schedule of Regulatory Assets and Liabilities [Line Items]
Program duration period			4 years
i-SRP | STRIDE | Atlanta Gas Light
Schedule of Regulatory Assets and Liabilities [Line Items]
Program duration period							4 years
Infrastructure replacement program, petitioned investment amount (more than)							$ 177,000,000
Scenario, Forecast | Atlanta Gas Light Company
Schedule of Regulatory Assets and Liabilities [Line Items]
Annual base rate adjustment, increase in base rate revenues									$ 22,000,000
Other Regulatory Liabilities [Member] | Subsequent Event | GEORGIA POWER CO
Schedule of Regulatory Assets and Liabilities [Line Items]
Customer Refund Liability, Noncurrent										$ 188,000,000