Contingencies and Regulatory Matters - Mississippi Power (Details) - MISSISSIPPI POWER CO - USD ($)			3 Months Ended	12 Months Ended
	Mar. 22, 2018	Feb. 07, 2018	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Mar. 23, 2018	Feb. 14, 2018	Sep. 30, 2017
Loss Contingencies [Line Items]
Performance evaluation plan, increase (decrease) in requested annual revenue		$ 26,000,000
Public utilities, requested return on equity, percentage		9.33%
Performance evaluation plan, equity ratio		55.00%
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
Environmental compliance overview plan, maximum annual increase in revenue, percentage										2.00%
Environmental compliance overview plan, maximum annual increase in revenue, amount										$ 17,000,000
Deferred revenue										$ 13,000,000
Fuel Cost Recovery					$ 6,000,000						$ 3,000,000
Ad valorem tax adjustment, annual rate increase, percentage									0.80%
Ad valorem tax adjustment, annual rate increase, amount									$ 7,000,000