Contingencies and Regulatory Matters - Other Matters (Details)	6 Months Ended
	Jun. 30, 2018 USD ($)
Southern Company Holdings Subsidiary
Loss Contingencies [Line Items]
Leveraged leases, balance sheet, investment in leveraged leases	$ 86,000,000
SOUTHERN Co GAS
Loss Contingencies [Line Items]
Impairment of long-lived assets	0
Natural Gas Storage
Loss Contingencies [Line Items]
Public utilities, property, plant and equipment, net	$ 111,000,000
Natural Gas Storage - Salt Dome Caverns
Loss Contingencies [Line Items]
Public utilities, property, plant and equipment, percentage of total	20.00%