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Retirement Benefits - Schedule of Health Care Cost Trend Rates (Details)
$ in Millions
12 Months Ended
Dec. 31, 2017
USD ($)
Defined Benefit Plan, Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rate [Abstract]  
Benefit obligation, 1 percent increase $ 132
Benefit obligation, 1 percent decrease 113
Service and interest costs, 1 percent increase 4
Service and interest costs, 1 percent decrease $ 3
Other postretirement benefit plans | Pre-65  
Defined Benefit Plan Disclosure [Line Items]  
Initial Cost Trend Rate 6.50%
Ultimate Cost Trend Rate 4.50%
Year That Ultimate Rate Is Reached 2026
Other postretirement benefit plans | Post-65 medical  
Defined Benefit Plan Disclosure [Line Items]  
Initial Cost Trend Rate 5.00%
Ultimate Cost Trend Rate 4.50%
Year That Ultimate Rate Is Reached 2026
Other postretirement benefit plans | Post-65 prescription  
Defined Benefit Plan Disclosure [Line Items]  
Initial Cost Trend Rate 10.00%
Ultimate Cost Trend Rate 4.50%
Year That Ultimate Rate Is Reached 2026
SOUTHERN POWER CO | Pre-65  
Defined Benefit Plan Disclosure [Line Items]  
Initial Cost Trend Rate 6.50%
Ultimate Cost Trend Rate 4.50%
Year That Ultimate Rate Is Reached 2026
SOUTHERN POWER CO | Post-65 medical  
Defined Benefit Plan Disclosure [Line Items]  
Initial Cost Trend Rate 5.00%
Ultimate Cost Trend Rate 4.50%
Year That Ultimate Rate Is Reached 2026
SOUTHERN POWER CO | Post-65 prescription  
Defined Benefit Plan Disclosure [Line Items]  
Initial Cost Trend Rate 10.00%
Ultimate Cost Trend Rate 4.50%
Year That Ultimate Rate Is Reached 2026
ALABAMA POWER CO  
Defined Benefit Plan, Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rate [Abstract]  
Benefit obligation, 1 percent increase $ 30
Benefit obligation, 1 percent decrease 26
Service and interest costs, 1 percent increase 1
Service and interest costs, 1 percent decrease $ 1
ALABAMA POWER CO | Other postretirement benefit plans | Pre-65  
Defined Benefit Plan Disclosure [Line Items]  
Initial Cost Trend Rate 6.50%
Ultimate Cost Trend Rate 4.50%
Year That Ultimate Rate Is Reached 2026
ALABAMA POWER CO | Other postretirement benefit plans | Post-65 medical  
Defined Benefit Plan Disclosure [Line Items]  
Initial Cost Trend Rate 5.00%
Ultimate Cost Trend Rate 4.50%
Year That Ultimate Rate Is Reached 2026
ALABAMA POWER CO | Other postretirement benefit plans | Post-65 prescription  
Defined Benefit Plan Disclosure [Line Items]  
Initial Cost Trend Rate 10.00%
Ultimate Cost Trend Rate 4.50%
Year That Ultimate Rate Is Reached 2026
GEORGIA POWER CO  
Defined Benefit Plan, Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rate [Abstract]  
Benefit obligation, 1 percent increase $ 59
Benefit obligation, 1 percent decrease 50
Service and interest costs, 1 percent increase 2
Service and interest costs, 1 percent decrease $ 2
GEORGIA POWER CO | Other postretirement benefit plans | Pre-65  
Defined Benefit Plan Disclosure [Line Items]  
Initial Cost Trend Rate 6.50%
Ultimate Cost Trend Rate 4.50%
Year That Ultimate Rate Is Reached 2026
GEORGIA POWER CO | Other postretirement benefit plans | Post-65 medical  
Defined Benefit Plan Disclosure [Line Items]  
Initial Cost Trend Rate 5.00%
Ultimate Cost Trend Rate 4.50%
Year That Ultimate Rate Is Reached 2026
GEORGIA POWER CO | Other postretirement benefit plans | Post-65 prescription  
Defined Benefit Plan Disclosure [Line Items]  
Initial Cost Trend Rate 10.00%
Ultimate Cost Trend Rate 4.50%
Year That Ultimate Rate Is Reached 2026
GULF POWER CO  
Defined Benefit Plan, Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rate [Abstract]  
Benefit obligation, 1 percent increase $ 4
Benefit obligation, 1 percent decrease 3
Service and interest costs, 1 percent increase 0
Service and interest costs, 1 percent decrease $ 0
GULF POWER CO | Other postretirement benefit plans | Pre-65  
Defined Benefit Plan Disclosure [Line Items]  
Initial Cost Trend Rate 6.50%
Ultimate Cost Trend Rate 4.50%
Year That Ultimate Rate Is Reached 2026
GULF POWER CO | Other postretirement benefit plans | Post-65 medical  
Defined Benefit Plan Disclosure [Line Items]  
Initial Cost Trend Rate 5.00%
Ultimate Cost Trend Rate 4.50%
Year That Ultimate Rate Is Reached 2026
GULF POWER CO | Other postretirement benefit plans | Post-65 prescription  
Defined Benefit Plan Disclosure [Line Items]  
Initial Cost Trend Rate 10.00%
Ultimate Cost Trend Rate 4.50%
Year That Ultimate Rate Is Reached 2026
MISSISSIPPI POWER CO  
Defined Benefit Plan, Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rate [Abstract]  
Benefit obligation, 1 percent increase $ 5
Benefit obligation, 1 percent decrease 5
Service and interest costs, 1 percent increase 0
Service and interest costs, 1 percent decrease $ 0
MISSISSIPPI POWER CO | Other postretirement benefit plans | Pre-65  
Defined Benefit Plan Disclosure [Line Items]  
Initial Cost Trend Rate 6.50%
Ultimate Cost Trend Rate 4.50%
Year That Ultimate Rate Is Reached 2026
MISSISSIPPI POWER CO | Other postretirement benefit plans | Post-65 medical  
Defined Benefit Plan Disclosure [Line Items]  
Initial Cost Trend Rate 5.00%
Ultimate Cost Trend Rate 4.50%
Year That Ultimate Rate Is Reached 2026
MISSISSIPPI POWER CO | Other postretirement benefit plans | Post-65 prescription  
Defined Benefit Plan Disclosure [Line Items]  
Initial Cost Trend Rate 10.00%
Ultimate Cost Trend Rate 4.50%
Year That Ultimate Rate Is Reached 2026
SOUTHERN Co GAS  
Defined Benefit Plan, Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rate [Abstract]  
Benefit obligation, 1 percent increase $ 11
Benefit obligation, 1 percent decrease 10
Service and interest costs, 1 percent increase 0
Service and interest costs, 1 percent decrease $ 0
SOUTHERN Co GAS | Other postretirement benefit plans | Pre-65  
Defined Benefit Plan Disclosure [Line Items]  
Initial Cost Trend Rate 6.40%
Ultimate Cost Trend Rate 4.50%
Year That Ultimate Rate Is Reached 2038
SOUTHERN Co GAS | Other postretirement benefit plans | Post-65 medical  
Defined Benefit Plan Disclosure [Line Items]  
Initial Cost Trend Rate 7.80%
Ultimate Cost Trend Rate 4.50%
Year That Ultimate Rate Is Reached 2038
SOUTHERN Co GAS | Other postretirement benefit plans | Post-65 prescription  
Defined Benefit Plan Disclosure [Line Items]  
Initial Cost Trend Rate 7.80%
Ultimate Cost Trend Rate 4.50%
Year That Ultimate Rate Is Reached 2038