v3.20.2

Accounting for derivative instruments and hedging activities (Details) $ in Millions, J in Billions |  | 1 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
May 15, 2020 MMBTU | Jan. 31, 2020 | Jun. 30, 2020 USD ($) J MMBTU contract item $ / 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 |
Gas purchase agreements | Morris Project |  |  |  |
Derivative instruments |  |  |  |
Notional amount, energy (in joules: Ontario, in Mmbtu: Orlando and Morris) | MMBTU | 700,000 |  |  |
Natural gas swaps |  |  |  |
Derivative instruments |  |  |  |
Notional amount, energy (in joules: Ontario, in Mmbtu: Orlando and Morris) | MMBTU |  |  | 14,500,000 | 16,300,000
Natural gas swaps | Orlando project |  |  |  |
Derivative instruments |  |  |  |
Amount of future natural gas purchases to fix the price (in Mmbtu) | MMBTU |  |  | 14,500,000 |
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 |  |  | $ 441.4 | $ 468.4
Derivative, Average Swaption Interest Rate |  |  | 2.21% |
Swaption interest rate (as a percent) |  |  | 4.71% |
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 |  |  | 345.0 |
Remaining aggregate principal amount |  |  | 345.0 |
Face amount of debt |  |  | $ 345.0 |

X
- Definition The number of forward contracts at end of period. + References No definition available. + Details
Name: | at_DerivativeForwardContracts
Namespace Prefix: | at_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- 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
Name: | at_DerivativePercentageOfEntitysShareOfExpectedNaturalGasPurchasesHedge
Namespace Prefix: | at_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition The price per Gigajoules ("Gj"). + References No definition available. + Details
Name: | at_NaturalGasPricePerGigajoules
Namespace Prefix: | at_
Data Type: | srt-types:perUnitItemType
Balance Type: | na
Period Type: | duration

X
- Definition Aggregate notional amount specified by the derivative for future natural gas purchases to effectively fix the price. + References No definition available. + Details
Name: | at_NotionalAmountOfFutureNaturalGasPurchasesWithFixedPrice
Namespace Prefix: | at_
Data Type: | num:energyItemType
Balance Type: | na
Period Type: | duration

X
- Definition Percentage points added to the reference rate to compute the variable rate on the debt instrument. + References No definition available. + Details
Name: | us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- 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 -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=120520924&loc=SL6031897-161870 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(16)) -URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910 Reference 3: 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(22)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -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=120398452&loc=d3e534808-122878 + Details
Name: | us-gaap_DebtInstrumentCarryingAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- 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 -Topic 835 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=119993939&loc=d3e28551-108399 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 55 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400 + Details
Name: | us-gaap_DebtInstrumentFaceAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- 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
Name: | us-gaap_DerivativeAverageSwaptionInterestRate
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Fixed interest rate related to the interest rate derivative. + References No definition available. + Details
Name: | us-gaap_DerivativeFixedInterestRate
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- 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
Name: | us-gaap_DerivativeFloorInterestRate
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- 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
Name: | us-gaap_DerivativeLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Nominal energy measure used to calculate payments on a derivative instrument. + References No definition available. + Details
Name: | us-gaap_DerivativeNonmonetaryNotionalAmountEnergyMeasure
Namespace Prefix: | us-gaap_
Data Type: | num:energyItemType
Balance Type: | na
Period Type: | duration

X
- 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 -URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5580258-113959 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1A -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5579245-113959 + Details
Name: | us-gaap_DerivativeNotionalAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | instant

X
- 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 -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1A -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5579245-113959 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5580258-113959 + Details
Name: | us-gaap_DerivativeNumberOfInstrumentsHeld
Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition Fixed interest rate that will be received or paid upon exercise of the interest rate swaption contract. + References No definition available. + Details
Name: | us-gaap_DerivativeSwaptionInterestRate
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Details
Name: | us-gaap_HedgingDesignationAxis=us-gaap_DesignatedAsHedgingInstrumentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_VariableRateAxis=at_AdjustedEurodollarRateMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_CapitalizedCostsOfUnprovedPropertiesExcludedFromAmortizationByPropertyOrProjectAxis=at_NipigonProjectMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DerivativeInstrumentRiskAxis=at_NaturalGasPurchaseCommitmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RangeAxis=srt_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_CapitalizedCostsOfUnprovedPropertiesExcludedFromAmortizationByPropertyOrProjectAxis=at_MorrisProjectMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DerivativeInstrumentRiskAxis=at_NaturalGasSwapsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_CapitalizedCostsOfUnprovedPropertiesExcludedFromAmortizationByPropertyOrProjectAxis=at_OrlandoProjectMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DerivativeInstrumentRiskAxis=at_NaturalGasSwapsDuringYearThreeMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateSwapMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | at_DerivativeSwaptionInterestRateAxis=at_DerivativeSwaptionInterestRateUntilFebruary152023Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_CapitalizedCostsOfUnprovedPropertiesExcludedFromAmortizationByPropertyOrProjectAxis=at_CadillacProjectMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | at_DerivativeSwaptionInterestRateAxis=at_DerivativeSwaptionInterestRateAfterFebruary152023Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DerivativeInstrumentRiskAxis=us-gaap_ForeignExchangeForwardMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_HedgingDesignationAxis=us-gaap_NondesignatedMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_ConsolidatedEntitiesAxis=srt_SubsidiariesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=us-gaap_SecuredDebtMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_LongtermDebtTypeAxis=at_LongTermDebtExcludingDebenturesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |