Contingencies and Regulatory Matters - Mississippi Power (Details) - USD ($)					3 Months Ended			6 Months Ended		12 Months Ended
	Jul. 27, 2018	May 08, 2018	Mar. 22, 2018	Feb. 07, 2018	Sep. 30, 2018	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Aug. 03, 2018
Loss Contingencies [Line Items]
Retail electric revenues						$ 3,740,000,000	$ 3,777,000,000	$ 7,308,000,000	$ 7,171,000,000
MISSISSIPPI POWER CO
Loss Contingencies [Line Items]
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%
Retail electric revenues						$ 212,000,000	$ 222,000,000	$ 406,000,000	$ 422,000,000
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
PEP Settlement Agreement | MISSISSIPPI POWER CO
Loss Contingencies [Line Items]
Public utilities, approved return on equity percentage								43.50%
Subsequent Event | MISSISSIPPI POWER CO
Loss Contingencies [Line Items]
Environmental compliance overview plan, maximum annual increase in revenue, amount															$ 17,000,000
Subsequent Event | PEP Settlement Agreement
Loss Contingencies [Line Items]
Public utilities, requested revenue requirement increase (decrease), compensation costs, amount	$ 5,500,000
Subsequent Event | PEP Settlement Agreement | MISSISSIPPI POWER CO
Loss Contingencies [Line Items]
Public utilities, approved revenue requirement increase (decrease), amount	$ 21,600,000
Public utilities, performance-adjusted allowed return on equity percentage	9.31%
Deferred income taxes	$ 44,000,000
Equity ratio, variance threshold	1.00%
Subsequent Event | Scenario, Forecast | PEP Settlement Agreement | MISSISSIPPI POWER CO
Loss Contingencies [Line Items]
Retail electric revenues					$ 5,000,000
Maximum | Subsequent Event | PEP Settlement Agreement | MISSISSIPPI POWER CO
Loss Contingencies [Line Items]
Public utilities, approved return on equity percentage	50.00%