Amount of obligation to return cash collateral under master netting arrangements that have not been offset against derivative assets.

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Summary of Significant Accounting Policies - Financial Instruments and Inventory at Lower of Cost or Market Adjustment (Details) - USD ($)	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2015	Dec. 31, 2014
Georgia Power [Member]
Inventory [Line Items]
Derivative collateral obligation to return cash	$ 0
Nicor Gas [Member]
Inventory [Line Items]
Excess of replacements or current costs over stated LIFO value	162,000,000
Inventory net	310,000,000
LIFO inventory amount	148,000,000
Southern Power [Member]
Inventory [Line Items]
Derivative collateral obligation to return cash	0
Alabama Power [Member]
Inventory [Line Items]
Derivative collateral obligation to return cash	0
Gulf Power [Member]
Inventory [Line Items]
Derivative collateral obligation to return cash	0
Successor [Member] \| Southern Company Gas [Member]
Inventory [Line Items]
Inventory valuation reserves	1,000,000
Successor [Member] \| Gas Marketing Services [Member] \| Southern Company Gas [Member]
Inventory [Line Items]
Inventory valuation reserves	0
Successor [Member] \| Wholesale Gas Services [Member] \| Southern Company Gas [Member]
Inventory [Line Items]
Inventory valuation reserves	1,000,000
Successor [Member] \| All Other [Member] \| Southern Company Gas [Member]
Inventory [Line Items]
Inventory valuation reserves	$ 0
Predecessor [Member] \| Southern Company Gas [Member]
Inventory [Line Items]
Inventory valuation reserves		$ 3,000,000	$ 23,000,000	$ 77,000,000
Predecessor [Member] \| Gas Marketing Services [Member] \| Southern Company Gas [Member]
Inventory [Line Items]
Inventory valuation reserves		0	3,000,000	4,000,000
Predecessor [Member] \| Wholesale Gas Services [Member] \| Southern Company Gas [Member]
Inventory [Line Items]
Inventory valuation reserves		3,000,000	19,000,000	73,000,000
Predecessor [Member] \| All Other [Member] \| Southern Company Gas [Member]
Inventory [Line Items]
Inventory valuation reserves		$ 0	$ 1,000,000	$ 0