Contingencies and Regulatory Matters - Southern Company Gas (Details) - USD ($) $ in Millions									1 Months Ended		12 Months Ended
	Dec. 02, 2019	Dec. 01, 2019	Jun. 01, 2019	Apr. 12, 2018	Mar. 26, 2018	Feb. 23, 2018	Feb. 15, 2018	Feb. 02, 2018	Dec. 31, 2017	Oct. 31, 2017	Dec. 31, 2019
Chattanooga Gas
Loss Contingencies [Line Items]
Public utilities, requested rate increase (decrease), amount							$ 7.0
Public utilities, requested return on equity, percentage							11.25%
Atlanta Gas Light
Loss Contingencies [Line Items]
Public utilities, requested rate increase (decrease), amount						$ 16.0			$ 22.0
Florida City Gas
Loss Contingencies [Line Items]
Public utilities, requested rate increase (decrease), amount										$ 19.0
Public utilities, interim rate										$ 5.0
Public utilities, requested rate increase (decrease), amended, amount					$ 11.5
Public utilities, requested return on equity, percentage					10.19%	54.00%
Public utilities, interim rate increase (decrease), percentage						3.00%
Nicor Gas
Loss Contingencies [Line Items]
Public utilities, requested rate increase (decrease), amended, amount								$ 44.0
Public utilities, requested return on equity, percentage								9.80%
Scenario, Forecast | Florida City Gas
Loss Contingencies [Line Items]
Public utilities, requested rate increase (decrease), amended, amount	$ 3.8	$ 1.3	$ 2.5
Public utilities, requested return on equity, percentage											56.00%
Public utilities, interim rate increase (decrease), percentage											2.00%
Subsequent Event | Elizabeth Gas
Loss Contingencies [Line Items]
Approved increase (decrease) in fuel rates amount				$ 11.0
Subsequent Event | Elkton Gas
Loss Contingencies [Line Items]
Approved increase (decrease) in fuel rates amount				$ 0.1
Minimum | Florida City Gas
Loss Contingencies [Line Items]
Public utilities, requested return on equity, percentage						10.55%
Maximum | Florida City Gas
Loss Contingencies [Line Items]
Public utilities, requested return on equity, percentage						10.95%