Aggregate notional amount specified by the derivative(s) as increments.

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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Period Type:	duration

- Definition

Interest rate that will be received or paid upon the exercise of the interest rate swaption contract after the addition of applicable margin in the fixed rate of the swap.

+ References

No definition available.

+ Details

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Namespace Prefix:	at_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The price per Gigajoules ("Gj").

+ References

No definition available.

+ Details

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Data Type:	us-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

The number of tranches of contracts to sell.

+ References

No definition available.

+ Details

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

- Definition

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

+ Details

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Balance Type:	na
Period Type:	instant

- 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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Period Type:	duration

- 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/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

+ Details

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

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

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5580258-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5579245-113959

+ Details

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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

- Definition

Foreign exchange rate used to translate amounts denominated in functional currency to reporting currency.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 830
-SubTopic 30
-Section S99
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6897108&loc=SL6897125-166521

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- 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) $ in Thousands, $ in Thousands, MMBTU in Millions, J in Billions		1 Months Ended	6 Months Ended	12 Months Ended
	Apr. 19, 2018	Jul. 31, 2017 CAD ($) item	Jun. 30, 2018 CAD ($) J MMBTU item $ / J	Dec. 31, 2017 CAD ($) MMBTU	Dec. 31, 2018 CAD ($)	Sep. 30, 2018	Jun. 30, 2018 USD ($) item	Jan. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 CAD ($)
Derivative instruments
Number of derivative contracts designated as cash flow hedges \| item			1				1
Applicable margin (as a percent)	3.00%
Adjusted Eurodollar Rate
Derivative instruments
Applicable margin (as a percent)			3.00%
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
Gas sales agreements \| Nipigon Project
Derivative instruments
Notional amount, energy (in joules: Ontario, in Mmbtu: Orlando and Morris) \| J			6,500
Price per Gj \| $ / J			2.75
Gas purchase and sales agreements
Derivative instruments
Notional amount							$ 700,000
Natural gas swaps
Derivative instruments
Notional amount, energy (in joules: Ontario, in Mmbtu: Orlando and Morris) \| MMBTU			14.1	9.9
Natural gas swaps \| Orlando project
Derivative instruments
Amount of future natural gas purchases to fix the price (in Mmbtu) \| MMBTU			14.1
Natural gas swaps during 2018 \| Orlando project
Derivative instruments
Percentage of the entity's share in required natural gas purchases hedge			90.00%
Natural gas swaps during 2019 \| 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							$ 664,600		$ 412,600
Minimum fixed interest rate (as a percent)			4.00%				4.00%
Swaption interest rate (as a percent)			1.34%				1.34%
Swaption all in one interest rate (as a percent)			4.34%				4.34%
Interest rate swaps \| New term loan facility
Derivative instruments
Notional amount							$ 457,700
Interest rate swaps \| Minimum \| Adjusted Eurodollar Rate
Derivative instruments
Minimum fixed interest rate (as a percent)			1.00%				1.00%
Interest rate swaps \| Swaption interest rate until February 15, 2019 \| Cadillac Project
Derivative instruments
Swaption interest rate (as a percent)			6.10%				6.10%
Interest rate swaps \| Swaption interest rate from February 16, 2019 to February 15, 2023 \| Cadillac Project
Derivative instruments
Swaption interest rate (as a percent)			6.30%				6.30%
Interest rate swaps \| Swaption interest rate after February 15, 2023 \| Cadillac Project
Derivative instruments
Swaption interest rate (as a percent)			6.40%				6.40%
Foreign currency forward contracts
Derivative instruments
Notional amount			$ 10,300	$ 25,000
Number of tranches \| item		3
Foreign Exchange Forward Rate 1.2481 \| Derivative instruments not designated as cash flow hedges
Derivative instruments
Notional amount		$ 10,000
Notional amount increments		$ 3,300	$ 3,300		$ 3,300
Foreign currency exchange rate		1.2481
Foreign Exchange Forward Rate 1.2943 \| Derivative instruments not designated as cash flow hedges
Derivative instruments
Notional amount		$ 10,000
Notional amount increments					$ 2,000
Foreign currency exchange rate		1.2943
Foreign Exchange Forward Rate 1.2196 \| Derivative instruments not designated as cash flow hedges
Derivative instruments
Notional amount										$ 5,000
Foreign currency exchange rate						1.2196
APLP Holdings \| Senior secured term loans
Derivative instruments
Face amount of debt							$ 700,000
APLP Holdings \| Interest rate swaps \| Adjusted Eurodollar Rate
Derivative instruments
Notional amount							457,700
APLP Holdings \| Interest rate swaps \| Adjusted Eurodollar Rate \| Senior secured term loans \| Long-term debt excluding debentures
Derivative instruments
Remaining aggregate principal amount							$ 490,000
APLP Holdings \| Interest rate swaps \| Derivative Swaption Interest Rate From June 30, 2018 To September 30, 2019
Derivative instruments
Notional amount								$ 100,000
APLP Holdings \| Interest rate swaps \| Derivative Swaption Interest Rate From June 30, 2018 To September 30, 2019 \| One-month LIBOR
Derivative instruments
Swaption interest rate (as a percent)								2.18%
APLP Holdings \| Interest rate swaps \| Derivative Swaption Interest Rate From October 1, 2019 To December 31, 2020
Derivative instruments
Notional amount								$ 200,000
APLP Holdings \| Interest rate swaps \| Derivative Swaption Interest Rate From October 1, 2019 To December 31, 2020 \| One-month LIBOR
Derivative instruments
Swaption interest rate (as a percent)								2.42%