Revenue from Contracts with Customers - Narrative (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Contract assets	$ 70	$ 70	$ 102
Contract liabilities	58	58	32
Revenue from contracts with customers recognized	11	27
Unregulated Distributed Generation
Disaggregation of Revenue [Line Items]
Contract assets	32	32	39
Contract liabilities	14	14	11
ALABAMA POWER CO
Disaggregation of Revenue [Line Items]
Contract assets	0	0	0
Contract liabilities	10	10	12
Revenue from contracts with customers recognized	0	0
GEORGIA POWER CO
Disaggregation of Revenue [Line Items]
Contract assets	30	30	58
Contract liabilities	26	26	7
Revenue from contracts with customers recognized	0	0
SOUTHERN Co GAS
Disaggregation of Revenue [Line Items]
Contract assets	0	0	0
Contract liabilities	1	1	2
Revenue from contracts with customers recognized	0	0
MISSISSIPPI POWER CO
Disaggregation of Revenue [Line Items]
Contract assets	0	0	0
Contract liabilities	7	7	$ 0
Revenue, remaining performance obligation	$ 0	$ 0

Amount, after allowance for credit loss, of right to consideration in exchange for good or service transferred to customer when right is conditioned on something other than passage of time.

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Amount of obligation to transfer good or service to customer for which consideration has been received or is receivable.

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Amount of transaction price allocated to performance obligation that has not been recognized as revenue.

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