Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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Amount of tax benefit for recognition of expense of award under share-based payment arrangement.

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Estimated period over which an employee is required to provide service in exchange for the equity-based payment award, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The amount of the tax credit carryforward, before tax effects, available to reduce future taxable income under enacted tax laws.

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STOCK COMPENSATION - Merger Stock Compensation (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	6 Months Ended	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award requisite service period (years)		3 years
Equity instrument, granted (in shares)		0.6
Total compensation cost for award recognized in income		$ 28	$ 27	$ 25
Total compensation cost for award recognized in income, tax benefit		$ 7	$ 7	$ 10
Parent Company \| Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity instrument, granted (in shares)	0.7
Equity instrument granted in period, weighted average grant date fair value (in dollars per share)	$ 53.83	$ 50.44	$ 43.81	$ 49.25
Merger-related expenses				$ 13
Southern Company Gas \| Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award requisite service period (years)		3 years
Southern Company Gas \| Change In Control Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage		33.00%
Total compensation cost for award recognized in income				12
Tax credit carryforward				$ 6