Stock Issued During Period, New Issues (Per Share)

No definition available.

Name:	ato_StockIssuedDuringPeriodNewIssuesPerShare
Namespace Prefix:	ato_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

The per share price of the Company's stock at which the contract holder of the freestanding contract has the right to purchase or sell the Company's stock at a future date.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 40
-Section 50
-Paragraph 5
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=6445032&loc=d3e90205-114008

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 480
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (f)(1)
-URI http://asc.fasb.org/extlink&oid=109262807&loc=d3e22047-110879

+ Details

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Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- 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.

+ References

No definition available.

+ Details

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Balance Type:	na
Period Type:	duration

- Definition

Current fair value of each cash settlement alternative for each freestanding forward contract.

+ References

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Number of shares as a settlement alternative for each freestanding forward contract.

+ References

+ Details

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Balance Type:	na
Period Type:	instant

- Definition

Settlement date of the forward contract indexed to issuer's equity, in CCYY-MM-DD format.

+ References

+ Details

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- Definition

The maximum number of shares that could be issued to net share settle a contract, if applicable. If a contract does not have a fixed or determinable maximum number of shares that may be required to be issued, disclose the fact that a potentially infinite number of shares could be issued to settle the contract.

+ References

+ Details

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Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of new stock issued during the period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(29))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
-Section 50
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-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21463-112644

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
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-Section S99
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-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
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-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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- Details

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- Details

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- Details

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- Details

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Shareholders' Equity Schedule of Forward Sales Executed (Details) - USD ($) $ / shares in Units, $ in Millions						3 Months Ended	6 Months Ended	9 Months Ended	12 Months Ended
	May 28, 2019	May 24, 2019	Feb. 19, 2019	Nov. 30, 2018	Nov. 30, 2017	Dec. 31, 2018	Mar. 31, 2019	Jun. 30, 2019	Sep. 30, 2018
Forward Contract Indexed to Issuer's Equity [Line Items]
Forward sales equity agreement, forward rate per share						$ 91.90	$ 93.08	$ 95.24	$ 0
Forward sales equity agreement, settlement in cash						$ 245.2	$ 403.9	$ 409.5	$ 0.0
Forward sales equity agreement, settlement in shares						2,668,464	4,338,973	4,299,836	0
At-The-Market
Forward Contract Indexed to Issuer's Equity [Line Items]
Forward sales equity agreement, shares		1,050,563	1,670,509
Forward sales equity agreement, forward rate per share		$ 101.41	$ 95.46
Common Stock Block Trade
Forward Contract Indexed to Issuer's Equity [Line Items]
Forward sales equity agreement, shares				2,668,464
Forward sales equity agreement, forward rate per share				$ 91.77
Stock Issued During Period, New Issues (in shares)	1,089,700			5,390,836	4,558,404
Stock Issued During Period, New Issues (Per Share)	$ 91.44
Forward Sales Equity, Maturity September 2020
Forward Contract Indexed to Issuer's Equity [Line Items]
Forward sales equity agreement, forward rate per share						$ 0	$ 0	$ 101.11	$ 0
Forward sales equity agreement, settlement date								Sep. 30, 2020
Forward sales equity agreement, settlement in cash						$ 0.0	$ 0.0	$ 106.2	$ 0.0
Forward sales equity agreement, settlement in shares						0	0	1,050,563	0
Forward Sales Equity, Maturity September 2020 \| At-The-Market
Forward Contract Indexed to Issuer's Equity [Line Items]
Forward sales equity agreement, shares		1,050,563	0
Forward sales equity agreement, forward rate per share		$ 101.41	$ 0
Forward Sales Equity, Maturity September 2020 \| Common Stock Block Trade
Forward Contract Indexed to Issuer's Equity [Line Items]
Forward sales equity agreement, shares				0
Forward sales equity agreement, forward rate per share				$ 0
Stock Issued During Period, New Issues (in shares)	0
Stock Issued During Period, New Issues (Per Share)	$ 0
Forward Sales Equity, Maturity March 2020
Forward Contract Indexed to Issuer's Equity [Line Items]
Forward sales equity agreement, forward rate per share						$ 91.90	$ 93.08	$ 93.34	$ 0
Forward sales equity agreement, settlement date								Mar. 31, 2020
Forward sales equity agreement, settlement in cash						$ 245.2	$ 403.9	$ 303.3	$ 0.0
Forward sales equity agreement, settlement in shares						2,668,464	4,338,973	3,249,273	0
Forward Sales Equity, Maturity March 2020 \| At-The-Market
Forward Contract Indexed to Issuer's Equity [Line Items]
Forward sales equity agreement, shares		0	1,670,509
Forward sales equity agreement, forward rate per share		$ 0	$ 95.46
Forward Sales Equity, Maturity March 2020 \| Common Stock Block Trade
Forward Contract Indexed to Issuer's Equity [Line Items]
Forward sales equity agreement, shares				2,668,464
Forward sales equity agreement, forward rate per share				$ 91.77
Stock Issued During Period, New Issues (in shares)	1,089,700
Stock Issued During Period, New Issues (Per Share)	$ 91.44