Financing - Other Long-Term Debt Narrative (Details) - USD ($)	3 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Nov. 30, 2018	Aug. 31, 2018
Debt Instrument [Line Items]
Long-term debt payable to affiliated trusts — variable rate (5.50% at 12/31/18) due 2042		$ 206,000,000	$ 206,000,000
Trust Preferred Securities Subject to Mandatory Redemption
Debt Instrument [Line Items]
Trust preferred securities		200,000,000	200,000,000
Alabama Power
Debt Instrument [Line Items]
Long-term debt payable to affiliated trusts — variable rate (5.50% at 12/31/18) due 2042		206,000,000	206,000,000
Mississippi Power
Debt Instrument [Line Items]
Revenue bond obligations face value		$ 270,000,000	270,000,000
Fixed stated interest rate of debt obligation		7.13%
Revenue bonds, current		$ 50,000,000	50,000,000
Mississippi Power | Plant Daniel Units 3 and 4
Debt Instrument [Line Items]
Significant acquisitions and disposals, acquisition costs, assumption of debt, at fair value		346,000,000
Fair value adjustment at date of purchase		76,000,000
Nicor Gas
Debt Instrument [Line Items]
First mortgage bonds		1,300,000,000	1,000,000,000
Nicor Gas | First Mortgage Bonds
Debt Instrument [Line Items]
Debt instrument, face amount		300,000,000
Nicor Gas | First Mortgage Bonds Issued August 2018 | First Mortgage Bonds
Debt Instrument [Line Items]
Debt instrument, face amount					$ 100,000,000
Nicor Gas | First Mortgage Bonds Issued November 2018 | First Mortgage Bonds
Debt Instrument [Line Items]
Debt instrument, face amount				$ 200,000,000
Pivotal Utility Holdings | Gas Facility Revenue Bonds
Debt Instrument [Line Items]
Debt redeemed	$ 200,000,000
Atlanta Gas Light | Medium-term Notes
Debt Instrument [Line Items]
Long-term debt		$ 159,000,000	$ 159,000,000