Business Acquisition, Conversion Ratio, Percentage of Underlying Stock Award Units

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- Definition

Percentage of Shares to Common Stock Based on Price Metrics and Performance Goals

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- Definition

Share Based Compensation, Shares Vested and Issued Each Year Employee Remains In Service At Each Vest Year, Percentage

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- Definition

Share-based Compensation Arrangement by Share-based Payment Award, Number of Performance Criteria

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- Definition

Share-based Compensation Arrangement by Share-based Payment Award, Performance Measure One, Percent of Award

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- Definition

Share-based Compensation Arrangement by Share-based Payment Award, Performance Measure Two, Percent of Award

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- Definition

Share-based Compensation Arrangement by Share-based Payment Award, Performance Period

+ References

No definition available.

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- Definition

Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.F)
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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
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-Section 50
-Paragraph 1
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-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (h)(1)(i)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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- Definition

Price of a single share of a number of saleable stocks paid or offered to be paid in a business combination.

+ References

No definition available.

+ Details

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- Definition

This element represents acquisition-related costs incurred to effect a business combination which costs have been expensed during the period. Such costs include finder's fees; advisory, legal, accounting, valuation, and other professional or consulting fees; general administrative costs, including the costs of maintaining an internal acquisitions department; and may include costs of registering and issuing debt and equity securities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 25
-Paragraph 23
-URI http://asc.fasb.org/extlink&oid=73531572&loc=d3e1043-128460

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- Definition

Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

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- Definition

Unrecognized cost of unvested share-based awards, other than options, awarded to employees as compensation.

+ References

No definition available.

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- Definition

Unrecognized cost of unvested options awarded to employees as compensation.

+ References

No definition available.

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- Definition

The cash inflow associated with the amount received from holders exercising their stock options. This item inherently excludes any excess tax benefit, which the entity may have realized and reported separately.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (j)
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Financing Activities
-URI http://asc.fasb.org/extlink&oid=6513228

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- Definition

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.

+ References

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- Definition

The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

+ References

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- Definition

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).

+ References

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- Definition

The weighted average fair value at grant date for nonvested equity-based awards issued 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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- Definition

The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

+ References

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- Definition

The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

+ References

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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- Definition

Number of additional shares authorized for issuance under an established share-based compensation plan.

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No definition available.

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- Definition

The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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- Definition

Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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- Definition

Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Carryforwards
-URI http://asc.fasb.org/extlink&oid=6506874

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Common Stock and Stock Compensation - Textual Southern Company Gas Awards (Details)	6 Months Ended		12 Months Ended
	Dec. 31, 2016 USD ($) shares	Jun. 30, 2016 USD ($) shares	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) performance_measure $ / shares shares	Dec. 31, 2015 USD ($) shares	Dec. 31, 2014 USD ($) shares	Jul. 01, 2016 $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total compensation cost for award recognized in income				$ 3,000,000	$ 6,000,000	$ 27,000,000
Tax credit carryforward	$ 1,800,000,000			$ 1,800,000,000
Shares vested and issued each year employee remains in service				0.33%
Aggregate intrinsic value for options outstanding	$ 195,000,000			$ 195,000,000
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity instrument, granted (in shares) \| shares				1,717,167
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number \| shares	3,224,539			3,224,539	2,480,392
Total compensation cost for award recognized in income				$ 96,000,000	$ 88,000,000	$ 33,000,000
Total unrecognized compensation cost related to award, weighted average period				22 months
Equity instruments, vested (in shares) \| shares				937,121
Equity instruments, forfeited (in shares) \| shares				35,899
Parent Company [Member] \| Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity instrument, granted (in shares) \| shares	742,461
Equity instrument granted in period, weighted average grant date fair value (in dollars per share) \| $ / shares				$ 53.83
Merger-related expenses				$ 13,000,000
Parent Company [Member] \| Scenario, Forecast [Member] \| Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total compensation cost for award recognized in income			$ 12,000,000
Southern Company Gas [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares vested and issued each year employee remains in service				0.33%
Southern Company Gas [Member] \| Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award requisite service period				3 years
Equity instrument, granted (in shares) \| shares		25,166			47,546	44,272
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number \| shares							65,042
Total compensation cost for award recognized in income				$ 13,000,000
Tax credit carryforward	$ 4,000,000			4,000,000
Compensation not yet recognized	12,000,000			$ 12,000,000
Total unrecognized compensation cost related to award, weighted average period				20 months
Southern Company Gas [Member] \| Change In Control Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation not yet recognized	20,000,000			$ 20,000,000
Total unrecognized compensation cost related to award, weighted average period				23 months
Southern Company Gas [Member] \| Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award requisite service period				3 years
Expiration period				10 years
Southern Company Gas [Member] \| Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of performance criteria \| performance_measure				2
Performance measure one, % of award					75.00%
Performance measure two, % of award					25.00%
Performance period					3 years
Southern Company Gas [Member] \| Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity instrument, granted (in shares) \| shares				303,618
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number \| shares					398,832
Equity instruments, vested (in shares) \| shares				699,960
Equity instruments, forfeited (in shares) \| shares				2,466
Southern Company Gas [Member] \| Successor [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Merger-related expenses	41,000,000
Southern Company Gas [Member] \| Successor [Member] \| Change In Control Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total compensation cost for award recognized in income				$ 4,000,000
Tax credit carryforward	$ 1,000,000			$ 1,000,000
Southern Company Gas [Member] \| Predecessor [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Merger-related expenses		$ 56,000,000			$ 44,000,000	$ 0
Compensation not yet recognized					0
Proceeds from stock options exercised		1,000,000			5,000,000
Aggregate intrinsic value for options outstanding		3,000,000			13,000,000	4,000,000
Southern Company Gas [Member] \| Predecessor [Member] \| Cash and Stock-based Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total compensation cost for award recognized in income		$ 24,000,000			$ 40,000,000	$ 24,000,000
Southern Company Gas [Member] \| Minimum [Member] \| Change In Control Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of shares to common stock based on price metrics and performance goals				0.00%
Southern Company Gas [Member] \| Maximum [Member] \| Change In Control Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of shares to common stock based on price metrics and performance goals				100.00%
Omnibus Performance Incentive Plan [Member] \| Southern Company Gas [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Remaining shares available for awards \| shares					359,586
Number of shares authorized \| shares					3,513,992
Long Term Incentive Plan [Member] \| Southern Company Gas [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Remaining shares available for awards \| shares					80,600
Long Term Incentive Plan [Member] \| Nicor Gas [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Remaining shares available for awards \| shares					1,514,116
Southern Company [Member] \| Southern Company Gas [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Business acquisition share price (in dollars per share) \| $ / shares							$ 66
Conversion ratio, percentage of underlying stock award units							125.00%