Consolidated Statements of Comprehensive Income (Loss) (Parenthetical) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Qualifying hedges, change in fair value, tax	$ (39)
Qualifying hedges, change in fair value, tax		$ (16)	$ 34
Qualifying hedges, reclassification adjustment, tax	19
Qualifying hedges, reclassification adjustment, tax		24	(37)
Pension and other postretirement benefit plans, gain (loss), tax	(31)	(2)	6
Reclassification adjustment for amounts included in net income, tax	1	5	(6)
Alabama Power
Qualifying hedges, change in fair value, tax	0
Qualifying hedges, change in fair value, tax		0	(1)
Qualifying hedges, reclassification adjustment, tax	2
Qualifying hedges, reclassification adjustment, tax		2	2
Georgia Power
Qualifying hedges, change in fair value, tax	(15)
Qualifying hedges, change in fair value, tax		0	0
Qualifying hedges, reclassification adjustment, tax	1
Qualifying hedges, reclassification adjustment, tax		1	1
Mississippi Power
Qualifying hedges, change in fair value, tax	0
Qualifying hedges, change in fair value, tax		(1)	(1)
Qualifying hedges, reclassification adjustment, tax	0
Qualifying hedges, reclassification adjustment, tax		0	1
Southern Power
Qualifying hedges, change in fair value, tax	(22)
Qualifying hedges, change in fair value, tax		(17)	39
Qualifying hedges, reclassification adjustment, tax	14
Qualifying hedges, reclassification adjustment, tax		19	(46)
Pension and other postretirement benefit plans, gain (loss), tax	(6)	2	0
Reclassification adjustment for amounts included in net income, tax	0	0	0
Southern Company Gas
Qualifying hedges, change in fair value, tax	(2)
Qualifying hedges, change in fair value, tax		2	(3)
Qualifying hedges, reclassification adjustment, tax	0
Qualifying hedges, reclassification adjustment, tax		(1)	0
Pension and other postretirement benefit plans, gain (loss), tax	(14)	0	0
Reclassification adjustment for amounts included in net income, tax	$ 0	$ 3	$ 0

Amount of tax expense (benefit) for (increase) decrease in value of benefit obligation for change in actuarial assumptions and increase (decrease) in value of plan assets from experience different from that assumed of defined benefit plan, that has not been recognized in net periodic benefit (cost) credit.

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- Definition

Amount, before reclassification, of tax expense (benefit) for gain (loss) from derivative instrument designated and qualifying as cash flow hedge included in assessment of hedge effectiveness.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=118930883&loc=d3e640-108580

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Amount of tax expense (benefit) for reclassification of gain (loss) from accumulated other comprehensive income (AOCI) for derivative instrument designated and qualifying as cash flow hedge included in assessment of hedge effectiveness.

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Amount of tax expense (benefit) of reclassification adjustment from accumulated other comprehensive income of accumulated gain (loss) realized from derivative instruments designated and qualifying as the effective portion of cash flow hedges and an entity's share of an equity investee's deferred hedging gain (loss).

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Amount of tax expense (benefit) of reclassification adjustment from accumulated other comprehensive income (loss) for gain (loss) of defined benefit plan.

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Amount of tax expense (benefit), before reclassification adjustments, related to increase (decrease) in accumulated gain (loss) from derivative instruments designated and qualifying as the effective portion of cash flow hedges and an entity's share of an equity investee's increase (decrease) in deferred hedging gain (loss).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=118930883&loc=d3e640-108580

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
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-Section 45
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-URI http://asc.fasb.org/extlink&oid=118930883&loc=SL7669646-108580

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