Contingencies and Regulatory Matters - Textual - General, Environmental Remediation, Nuclear Fuel Costs, FERC Matters (Details) $ in Millions										1 Months Ended				2 Months Ended	12 Months Ended
	Feb. 02, 2017	Jan. 01, 2017 USD ($)	Dec. 13, 2016	Aug. 17, 2016 USD ($)	Jun. 09, 2016 USD ($)	May 01, 2016 USD ($)	Mar. 31, 2016	Apr. 01, 2015 USD ($)	May 01, 2014 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Jul. 31, 2015 USD ($)	Mar. 31, 2015 USD ($)	Mar. 15, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) state utility pipeline defendant site	Dec. 31, 2014 USD ($) installment	Dec. 31, 2013	Dec. 20, 2016 USD ($)	Dec. 31, 2015 USD ($)
Mississippi Power [Member]
Loss Contingencies [Line Items]
Regulatory Asset, Deferral Period																		10 years
Finite-lived Asset, Remaining Life																		30 years
AFUDC Cost								$ 14.0
Increase in Base Rate Under Cost Based Electric Tariff Due to Settlement						$ 7.0			$ 10.0
Public Utilities, Approved Rate Increase (Decrease), Amount				$ 18.0
Over Recovered Fuel Cost																				$ 71.0
Georgia Power [Member]
Loss Contingencies [Line Items]
Environmental Exit Costs, Assets Previously Disposed, Liability for Remediation																$ 17.0
Claims Awarded to Companies Related to Nuclear Fuel Disposal Litigation													$ 18.0
Loss Contingency, Estimate of Possible Loss																			$ 240.0
Gulf Power [Member]
Loss Contingencies [Line Items]
Environmental Exit Costs, Assets Previously Disposed, Liability for Remediation																$ 44.0
Southern Company Gas [Member]
Loss Contingencies [Line Items]
Number of states in which entity operates | state																7
Number of natural gas distribution utilities | utility																7
Nicor Gas [Member]
Loss Contingencies [Line Items]
Loss Contingency, Damages Sought, Value											$ 0.3
Alabama Power [Member]
Loss Contingencies [Line Items]
Claims Awarded to Companies Related to Nuclear Fuel Disposal Litigation													$ 26.0
Over Recovered Fuel Cost																$ 76.0				238.0
Virginia Natural Gas
Loss Contingencies [Line Items]
Period to File Subsequent Rate Request			60 days
Scenario, Forecast [Member] | Mississippi Power [Member]
Loss Contingencies [Line Items]
AFUDC Cost														$ 14.0
Public Utilities, Approved Rate Increase (Decrease), Amount															$ 22.0
Successor [Member] | Southern Company Gas [Member]
Loss Contingencies [Line Items]
Environmental Exit Costs, Costs Accrued to Date																426.0
Environmental Exit Costs, Costs Accrued to Date, Incur Next Twelve Months																69.0
Predecessor [Member] | Southern Company Gas [Member]
Loss Contingencies [Line Items]
Increase (Decrease) in Insurance Settlements Receivable																	$ 77.0
Insurance Settlements Receivable Number Of Installments | installment																	2
Proceeds from Insurance Settlement, Operating Activities												$ 32.0					$ 45.0
Location One [Member] | Successor [Member] | Southern Company Gas [Member]
Loss Contingencies [Line Items]
Environmental Exit Costs, Assets Previously Disposed, Liability for Remediation																5.0
Gas pipeline | Southern Company Gas [Member]
Loss Contingencies [Line Items]
Approved Infrastructure Replacement Program																780.0
Under Recovered Regulatory Clause Revenues and Other Current Liabilities [Member] | Gulf Power [Member]
Loss Contingencies [Line Items]
Environmental Exit Costs, Assets Previously Disposed, Liability for Remediation																4.0
Other Regulatory Assets, Deferred and Other Deferred Credits and Liabilities [Member] | Gulf Power [Member]
Loss Contingencies [Line Items]
Environmental Exit Costs, Assets Previously Disposed, Liability for Remediation																40.0
Kemper IGCC [Member] | Mississippi Power [Member]
Loss Contingencies [Line Items]
AFUDC Cost								$ 11.0
Period of Amortization of Regulatory Assets							36 months
MRA Revenue [Member] | Mississippi Power [Member]
Loss Contingencies [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount										$ (11.0)
Over Recovered Fuel Cost																$ 13.0				$ 24.0
MB Revenue [Member] | Mississippi Power [Member]
Loss Contingencies [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount										$ (1.0)
Subsequent Event [Member] | Gulf Power [Member]
Loss Contingencies [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount		$ (41.0)
Subsequent Event [Member] | Traditional Operating Companies and Southern Power [Member]
Loss Contingencies [Line Items]
Period to File Subsequent Rate Request	30 days
Subsequent Event [Member] | MRA Revenue [Member] | Mississippi Power [Member]
Loss Contingencies [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount		$ 10.0
Pending Litigation [Member] | Southern Company Gas [Member]
Loss Contingencies [Line Items]
Number of states in which entity operates | state																1
Number of natural gas distribution utilities | utility																1
Loss Contingency, Number of Defendants | defendant																1
Pending Litigation [Member] | Kemper IGCC [Member] | Mississippi Power [Member]
Loss Contingencies [Line Items]
Loss Contingency, Damages Sought, Value					$ 100.0
Minimum [Member] | Pending Litigation [Member] | Southern Company Gas [Member]
Loss Contingencies [Line Items]
Loss Contingency, Estimate of Possible Loss																$ 11.0
Manufactured Gas Plants [Member] | Southern Company Gas [Member]
Loss Contingencies [Line Items]
Number of states in which entity operates | state																5
# of sites | site																46
Midstream Operations [Member] | Gas pipeline | Southern Company Gas [Member]
Loss Contingencies [Line Items]
Number of Gas Construction Projects | pipeline																3