Summary of Significant Accounting Policies - Financial Instruments and Inventory at Lower of Cost or Market Adjustment (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2015
Inventory [Line Items]
Derivative collateral obligation to return cash	$ 0
ALABAMA POWER CO
Inventory [Line Items]
Derivative collateral obligation to return cash	0
GULF POWER CO
Inventory [Line Items]
Derivative collateral obligation to return cash	0
SOUTHERN POWER CO
Inventory [Line Items]
Derivative collateral obligation to return cash	0	$ 0
SOUTHERN Co GAS
Inventory [Line Items]
Inventory valuation reserves	2,000,000	1,000,000
Derivative collateral obligation to return cash	0
Nicor Gas
Inventory [Line Items]
Inventory net	264,000,000
LIFO inventory amount	148,000,000
Gas Marketing Services | SOUTHERN Co GAS
Inventory [Line Items]
Inventory valuation reserves	0	0
Wholesale Gas Services | SOUTHERN Co GAS
Inventory [Line Items]
Inventory valuation reserves	2,000,000	1,000,000
All Other | SOUTHERN Co GAS
Inventory [Line Items]
Inventory valuation reserves	$ 0	$ 0
Predecessor | SOUTHERN Co GAS
Inventory [Line Items]
Inventory valuation reserves			$ 3,000,000	$ 23,000,000
Predecessor | Gas Marketing Services | SOUTHERN Co GAS
Inventory [Line Items]
Inventory valuation reserves			0	3,000,000
Predecessor | Wholesale Gas Services | SOUTHERN Co GAS
Inventory [Line Items]
Inventory valuation reserves			3,000,000	19,000,000
Predecessor | All Other | SOUTHERN Co GAS
Inventory [Line Items]
Inventory valuation reserves			$ 0	$ 1,000,000