Regulatory Matters - GAS FERC Regulatory Matters (Details) - Pipelines - Southern Company Gas $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) pipeline	Oct. 31, 2017 USD ($)
Schedule of Regulatory Assets and Liabilities [Line Items]
Number of gas construction projects | pipeline	2
Percentage of construction costs responsible to pay	5.00%
Investment	$ 83
Minimum
Schedule of Regulatory Assets and Liabilities [Line Items]
Estimated project costs	7,000	$ 6,000
Maximum
Schedule of Regulatory Assets and Liabilities [Line Items]
Estimated project costs	$ 7,800	$ 6,500