Period between anniversaries from the date the option to purchase is first exercisable.

No definition available.

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- Definition

Sales price of plant under PPA accounted for as a lease per agreement.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
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- Definition

Remaining lease term of lessor operating lease, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

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Balance Type:	na
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- Definition

The number of PPAs accounted for as operating leases under lessor.

+ References

No definition available.

+ Details

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Balance Type:	na
Period Type:	instant

- Definition

Represents the number of power generation projects which are in operation.

+ References

No definition available.

+ Details

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Namespace Prefix:	at_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

n/a

+ References

No definition available.

+ Details

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- Definition

Indicates (true false) whether practical expedients are elected for not reassessing whether any expired or existing contract is or contains lease, lease classification for expired or existing lease, and initial direct cost for any existing lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 10
-Section 65
-Paragraph 1
-Subparagraph (j)
-URI http://asc.fasb.org/extlink&oid=121329987&loc=SL77916155-209984

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 10
-Section 65
-Paragraph 1
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=121329987&loc=SL77916155-209984

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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.

+ References

No definition available.

+ Details

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Balance Type:	na
Period Type:	duration

- Definition

Indicates (true false) whether lessor has option to extend operating lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 30
-Section 50
-Paragraph 3
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=121576215&loc=SL77919359-209981

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:booleanItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of operating lease income from lease payments paid and payable to lessor. Excludes variable lease payments not included in measurement of lease receivable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 270
-SubTopic 10
-Section 50
-Paragraph 6A
-URI http://asc.fasb.org/extlink&oid=121640914&loc=SL77927221-108306

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 30
-Section 50
-Paragraph 5
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121576215&loc=SL77919372-209981

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
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Balance Type:	na
Period Type:	duration

- 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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Leases - PPA Leases (Details) $ in Millions	1 Months Ended	3 Months Ended		9 Months Ended
Leases - PPA Leases (Details) $ in Millions	May 31, 2019 USD ($)	Sep. 30, 2020 USD ($) project agreement	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) project agreement	Sep. 30, 2019 USD ($)
Lessor, Lease, Description [Line Items]
Lease, Practical Expedients, Package [true false]				true
Number of PPAs accounted for as operating leases \| agreement		10		10
Number of project which are in operation \| project		21		21
Lessor, Operating Lease, Existence of Option to Extend [true false]				false
Operating Lease, Lease Income [Abstract]
Rental income from operating leases		$ 35.2	$ 42.3	$ 115.6	$ 133.4
Minimum
Lessor, Lease, Description [Line Items]
Remaining lease term				19 months
Maximum
Lessor, Lease, Description [Line Items]
Remaining lease term				23 years
Manchief
Operating Lease Lessor Sale Of Plant [Abstract]
Sales price of Manchief plant	$ 45.2
Mamquam \| BC Hydro
Operating Lease Lessor Sale Of Plant [Abstract]
Period of time between the anniversaries the option to purchase will become exercisable after the option is first exercisable				5 years
Solid Fuel
Operating Lease, Lease Income [Abstract]
Rental income from operating leases		19.0	26.1	$ 51.1	64.4
Natural Gas
Operating Lease, Lease Income [Abstract]
Rental income from operating leases		7.0	6.5	19.0	18.4
Hydroelectric
Operating Lease, Lease Income [Abstract]
Rental income from operating leases		$ 9.2	$ 9.7	$ 45.5	$ 50.6