Contingencies and Regulatory Matters - Mississippi Power (Details) - MISSISSIPPI POWER CO - USD ($)					3 Months Ended	9 Months Ended	12 Months Ended
	Jul. 27, 2018	May 08, 2018	Mar. 22, 2018	Feb. 07, 2018	Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Aug. 03, 2018
Loss Contingencies [Line Items]
Refund due to customers											$ 5,000,000
Performance evaluation plan, prior year lookback, surcharge (refund)			$ 0					$ 5,000,000	$ 5,000,000	$ 0	$ 0
Public utilities, interim rate increase (decrease), percentage						4.00%	0.00%	0.00%
Public utilities, interim rate						$ 38,000,000
Public utilities, requested revenue requirement increase (decrease), amount				$ 26,000,000
Public utilities, requested return on equity, percentage				9.33%
Performance evaluation plan, equity ratio				55.00%
Increase retail base rate, annual base revenue		$ 3,000,000
Ad valorem tax adjustment, annual rate increase, percentage		0.80%
Ad valorem tax adjustment, annual rate increase, amount		$ 7,000,000
Environmental compliance overview plan, maximum annual increase in revenue, amount												$ 17,000,000
Over (under) recovered retail fuel costs					$ 13,000,000	$ 13,000,000	$ (6,000,000)
PEP Settlement Agreement
Loss Contingencies [Line Items]
Public utilities, approved revenue requirement increase (decrease), amount	$ 21,600,000
Amount subsequently approved for recovery	$ 2,000,000
Public utilities, performance-adjusted allowed return on equity percentage	9.31%
Public utilities, approved return on equity percentage						45.00%
Deferred income taxes	$ 44,000,000
Equity ratio, variance threshold	1.00%
Retail electric revenues					5,000,000
PEP Settlement Agreement | Maximum
Loss Contingencies [Line Items]
Public utilities, approved return on equity percentage	50.00%
Other Regulatory Assets | PEP Settlement Agreement
Loss Contingencies [Line Items]
Regulatory asset					3,000,000	$ 3,000,000
Other Regulatory Assets
Loss Contingencies [Line Items]
Regulatory liabilities					2,000,000	2,000,000
Other Regulatory Assets | PEP Settlement Agreement
Loss Contingencies [Line Items]
Regulatory liabilities					$ 5,000,000	$ 5,000,000