The aggregate amount of write-downs for impairments recognized during the period for long lived assets held for use (including those held for disposal by means other than sale).

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Period Type:	duration

Contingencies and Regulatory Matters - Other Matters (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2018 USD ($) cavern	Dec. 31, 2017 USD ($)
Southern Company Holdings Subsidiary
Loss Contingencies [Line Items]
Lease receivable	$ 86,000,000
Number of salt dome caverns \| cavern	2
SOUTHERN Co GAS
Loss Contingencies [Line Items]
Impairment of long-lived assets		$ 0
Natural Gas, Storage
Loss Contingencies [Line Items]
Property, plant and equipment, net book value	$ 110,000,000
Natural Gas Storage - Salt Dome Caverns
Loss Contingencies [Line Items]
Percentage of total property, plant and equipment	20.00%