Public Utilities, Approved Customer Credit, Amount

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Amount awarded from other party in judgment or settlement of litigation.

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Amount of public utility's approved rate increase (decrease) by regulatory agency.

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Percentage of public utility's approved return on equity.

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Amount of public utility's interim rate increase (decrease) allowed while waiting final decision of regulatory agency.

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Percentage of public utility's interim rate increase (decrease) allowed while waiting final decision of regulatory agency.

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- Definition

Amount of public utility's amended requested rate increase (decrease) with regulatory agency.

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Amount of public utility's requested rate increase (decrease) with regulatory agency.

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Percentage of public utility's requested return on equity.

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Contingencies and Regulatory Matters - Southern Company Gas (Details) - USD ($) $ in Millions								1 Months Ended	3 Months Ended
	Jun. 22, 2018	May 15, 2018	May 02, 2018	Mar. 28, 2018	Mar. 26, 2018	Feb. 23, 2018	Feb. 15, 2018	Oct. 31, 2017	Mar. 31, 2018
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%
Public utilities, interim rate increase (decrease), percentage		4.00%
Public utilities, approved return on equity percentage		55.00%
Atlanta Gas Light
Loss Contingencies [Line Items]
Public utilities, requested rate increase (decrease), amount						$ 16.0
Customer credit		$ 8.0
Atlanta Gas Light \| Pipeline Replacement Program
Loss Contingencies [Line Items]
Litigation settlement awarded from other party									$ 7.0
Nicor Gas
Loss Contingencies [Line Items]
Approved increase (decrease) in fuel rates amount			$ (44.0)
Public utilities, approved return on equity percentage			9.80%
Chattanooga Gas
Loss Contingencies [Line Items]
Public utilities, requested rate increase (decrease), amount							$ (7.0)
Public utilities, requested return on equity, percentage							11.25%
Elizabeth Gas
Loss Contingencies [Line Items]
Approved increase (decrease) in fuel rates amount	$ (12.0)
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%