The number of forward contracts at end of period.

No definition available.

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- Definition

Represents the percentage of the entity's share in the expected natural gas purchase which was hedged by swap agreement.

+ References

No definition available.

+ Details

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

- Definition

The price per Gigajoules ("Gj").

+ References

No definition available.

+ Details

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Data Type:	srt-types:perUnitItemType
Balance Type:	na
Period Type:	duration

- Definition

Aggregate notional amount specified by the derivative for future natural gas purchases to effectively fix the price.

+ References

No definition available.

+ Details

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Namespace Prefix:	at_
Data Type:	num:energyItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

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- Definition

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
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-SubTopic 20
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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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+ Details

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

- Definition

Average fixed interest rate that will be received or paid upon exercise of a group of interest rate swaption contracts.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Fixed interest rate related to the interest rate derivative.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Floor rate on an interest rate derivative such as an interest rate floor or collar. If market rates falls below the floor rate, a payment or receipt is triggered on the contract.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

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

- Definition

Nominal energy measure used to calculate payments on a derivative instrument.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:energyItemType
Balance Type:	na
Period Type:	duration

- Definition

Nominal or face amount used to calculate payment on derivative.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
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Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
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-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5579245-113959

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Definition

The number of derivative instruments of a particular group held by the entity.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Fixed interest rate that will be received or paid upon exercise of the interest rate swaption contract.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Details

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- Details

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- Details

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- Details

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Accounting for derivative instruments and hedging activities (Details) MMBTU in Millions, $ in Millions, J in Billions	1 Months Ended	3 Months Ended	12 Months Ended
	Jan. 31, 2020	Mar. 31, 2020 USD ($) J MMBTU item contract $ / J	Dec. 31, 2019 USD ($) MMBTU
Derivative instruments designated as cash flow hedges
Derivative instruments
Number of derivative contracts designated as cash flow hedges \| item		1
Adjusted Eurodollar Rate
Derivative instruments
Applicable margin (as a percent)		2.50%
Nipigon Project
Derivative instruments
Notional amount, energy (in joules: Ontario, in Mmbtu: Orlando and Morris) \| J		6,500
Gas purchase agreements \| Nipigon Project
Derivative instruments
Price per Gj \| $ / J		4.57
Gas purchase agreements \| Nipigon Project \| Minimum
Derivative instruments
Notional amount, energy (in joules: Ontario, in Mmbtu: Orlando and Morris) \| J		6,500
Natural gas swaps
Derivative instruments
Notional amount, energy (in joules: Ontario, in Mmbtu: Orlando and Morris) \| MMBTU		15.4	16.3
Natural gas swaps \| Orlando project
Derivative instruments
Amount of future natural gas purchases to fix the price (in Mmbtu) \| MMBTU		15.4
Natural gas swaps during 2016 \| Orlando project
Derivative instruments
Percentage of the entity's share in required natural gas purchases hedge		100.00%
Interest rate swaps
Derivative instruments
Notional amount		$ 346.6	$ 468.4
Derivative, Average Swaption Interest Rate		2.03%
Swaption interest rate (as a percent)		4.53%
Interest rate swaps \| Adjusted Eurodollar Rate
Derivative instruments
Derivative, Floor Interest Rate		1.00%
Interest rate swaps \| Minimum
Derivative instruments
Minimum fixed interest rate (as a percent)		3.50%
Interest rate swaps \| Swaption interest rate until February 15, 2023 \| Cadillac Project
Derivative instruments
Swaption interest rate (as a percent)		6.30%
Interest rate swaps \| Swaption interest rate after February 15, 2023 \| Cadillac Project
Derivative instruments
Swaption interest rate (as a percent)		6.40%
Foreign currency forward contracts \| Derivative instruments not designated as cash flow hedges
Derivative instruments
Number of forward contracts \| contract		0
APLP Holdings \| Senior secured term loans
Derivative instruments
Face amount of debt		$ 700.0
Applicable margin (as a percent)	2.50%
APLP Holdings \| Interest rate swaps \| Adjusted Eurodollar Rate \| Senior secured term loans \| Long-term debt excluding debentures
Derivative instruments
Notional amount		360.0
Remaining aggregate principal amount		329.4
Face amount of debt		$ 360.0
APLP Holdings \| Interest rate swaps \| Derivative Swaption Interest Rate from Mar 31 2020 to December 31 2021 \| One-month LIBOR \| New term loan facility
Derivative instruments
Swaption interest rate (as a percent)		1.39%
APLP Holdings \| Interest rate swaps \| Derivative Swaption Interest Rate from Mar 31 2020 to December 31 2021 \| Minimum \| New term loan facility
Derivative instruments
Notional amount		$ 3.1
APLP Holdings \| Interest rate swaps \| Derivative Swaption Interest Rate from Mar 31 2020 to December 31 2021 \| Maximum \| New term loan facility
Derivative instruments
Notional amount		$ 100.0