v3.20.1

Long-term debt (Details) $ in Millions |  | 1 Months Ended | 3 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | --- | ---
Mar. 18, 2020 USD ($) | Jan. 31, 2020 USD ($) | Mar. 31, 2020 USD ($) | Mar. 31, 2020 CAD ($) | Mar. 31, 2020 USD ($) | Mar. 17, 2020 USD ($) | Jan. 31, 2020 CAD ($) | Dec. 31, 2019 USD ($)
Long-term debt |  |  |  |  |  |  |  |
Less: unamortized discount |  |  |  |  | $ (5,200,000) |  |  | $ (5,800,000)
Less: unamortized deferred financing costs |  |  |  |  | (5,200,000) |  |  | (4,700,000)
Less: current maturities |  |  |  |  | (78,500,000) |  |  | (76,400,000)
Total long-term debt |  |  |  |  | 436,300,000 |  |  | 473,500,000
Current maturities |  |  |  |  | (78,500,000) |  |  | (76,400,000)
Term loan maturity |  | 2 years |  |  |  |  |  |
Payments of Financing Costs |  |  | $ 1,500,000 |  |  |  |  |
Senior secured term loans | APLP Holdings |  |  |  |  |  |  |  |
Long-term debt |  |  |  |  |  |  |  |
Minimum percentage of variable rate base |  | 1.00% |  |  |  |  |  |
Applicable margin (as a percent) |  | 2.50% |  |  |  |  |  |
Write-off of deferred financing costs |  | $ 500,000 |  |  |  |  |  |
Debt repriced |  |  |  |  |  |  | $ 380.0 |
Interest rate margin reduction due to repricing, as a percent |  | 0.25% |  |  |  |  |  |
Interest rate margin additional reduction due to repricing, as a percent |  | 0.25% |  |  |  |  |  |
Payments of Financing Costs |  | $ 600,000 |  |  |  |  |  |
Senior unsecured notes, due June 2036 |  |  |  |  |  |  |  |
Long-term debt |  |  |  |  |  |  |  |
Long-term Debt, Gross |  |  |  | $ 210.0 |  |  |  |
Cadillac term loan, due 2025 |  |  |  |  |  |  |  |
Long-term debt |  |  |  |  |  |  |  |
Debt service coverage ratio |  |  | 1.20% |  |  |  |  |
Debt service coverage ratio, consecutive term |  |  | 4 |  |  |  |  |
Debt service coverage projected ratio |  |  | 1.20% |  |  |  |  |
Debt service coverage projected ratio, consecutive term |  |  | 4 |  |  |  |  |
Revolver | APLP Holdings |  |  |  |  |  |  |  |
Long-term debt |  |  |  |  |  |  |  |
Line of Credit Facility, Amount Outstanding |  |  |  |  | 0 |  |  |
Borrowing capacity | $ 180,000,000 |  |  |  |  | $ 200,000,000 |  |
Payments of Financing Costs | 900,000 |  |  |  |  |  |  |
Letters of credit | APLP Holdings |  |  |  |  |  |  |  |
Long-term debt |  |  |  |  |  |  |  |
Line of Credit Facility, Amount Outstanding |  |  |  |  | 78,100,000 |  |  |
Maximum | Senior secured term loans | APLP Holdings |  |  |  |  |  |  |  |
Long-term debt |  |  |  |  |  |  |  |
Leverage ratio as defined in Credit Agreement |  |  |  |  |  |  | 2.75 |
Maximum | Revolver | APLP Holdings |  |  |  |  |  |  |  |
Long-term debt |  |  |  |  |  |  |  |
Borrowing capacity | 210,000,000 |  |  |  |  |  |  |
Upsizing borrowing capacity | $ 30,000,000 |  |  |  |  |  |  |
London Interbank Offered Rate (LIBOR) | Senior secured term loan facility, due 2025 |  |  |  |  |  |  |  |
Long-term debt |  |  |  |  |  |  |  |
Minimum percentage of variable rate base |  |  | 1.00% |  |  |  |  |
Long-term debt excluding debentures | Senior secured term loan facility, due 2025 |  |  |  |  |  |  |  |
Long-term debt |  |  |  |  |  |  |  |
Long-term Debt, Gross |  |  |  |  | 360,000,000 |  |  | 380,000,000
Current maturities |  |  |  |  | (75,500,000) |  |  | (72,500,000)
Long-term debt excluding debentures | Senior unsecured notes, due June 2036 |  |  |  |  |  |  |  |
Long-term debt |  |  |  |  |  |  |  |
Long-term Debt, Gross |  |  |  |  | $ 148,000,000 |  |  | 161,700,000
Interest rate (as a percent) |  |  |  | 5.95% | 5.95% |  |  |
Long-term debt excluding debentures | Cadillac term loan, due 2025 |  |  |  |  |  |  |  |
Long-term debt |  |  |  |  |  |  |  |
Long-term Debt, Gross |  |  |  |  | $ 17,200,000 |  |  | 18,700,000
Current maturities |  |  |  |  | $ (3,000,000) |  |  | $ (3,900,000)
Long-term debt excluding debentures | London Interbank Offered Rate (LIBOR) | Senior secured term loan facility, due 2025 |  |  |  |  |  |  |  |
Long-term debt |  |  |  |  |  |  |  |
Applicable margin (as a percent) |  |  | 2.50% |  |  |  |  |
Long-term debt excluding debentures | London Interbank Offered Rate (LIBOR) | Cadillac term loan, due 2025 |  |  |  |  |  |  |  |
Long-term debt |  |  |  |  |  |  |  |
Applicable margin (as a percent) |  |  | 1.61% |  |  |  |  |

X
- Definition Represents the additional maturity term of debt instrument. + References No definition available. + Details
Name: | at_DebtInstrumentAdditionalTerm
Namespace Prefix: | at_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the ratio of consolidated cash available to meet the debt obligations of the company, which is necessary to be maintained under the terms of the debt covenants. + References No definition available. + Details
Name: | at_DebtInstrumentCovenantConsolidatedDebtServiceCoverageRatio
Namespace Prefix: | at_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the required consecutive period the ratio of consolidated cash available must be met for the debt obligations of the company, which is necessary to be maintained under the terms of the debt covenants. + References No definition available. + Details
Name: | at_DebtInstrumentCovenantConsolidatedDebtServiceCoverageRatioTerm
Namespace Prefix: | at_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the projected ratio of consolidated cash available to meet the debt obligations of the company, which is necessary to be maintained under the terms of the debt covenants. + References No definition available. + Details
Name: | at_DebtInstrumentCovenantConsolidatedProjectedDebtServiceCoverageRatio
Namespace Prefix: | at_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the required consecutive period the projected ratio of consolidated cash available must be met for the debt obligations of the company, which is necessary to be maintained under the terms of the debt covenants. + References No definition available. + Details
Name: | at_DebtInstrumentCovenantConsolidatedProjectedDebtServiceCoverageRatioTerm
Namespace Prefix: | at_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | duration

X
- Definition The percentage of additional reduction on the interest rate margin due to repricing the debt instrument effective in the event the Leverage Ratio is as defined in the Credit Agreement . + References No definition available. + Details
Name: | at_DebtInstrumentInterestRateAdditionalReductionDueToRePricing
Namespace Prefix: | at_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition The percentage of reduction on the interest rate margin due to repricing the debt instrument. + References No definition available. + Details
Name: | at_DebtInstrumentInterestRateReductionDueToRePricing
Namespace Prefix: | at_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the minimum reference rate for variable rate of debt instrument, such as LIBOR or the US Treasury rate and the maturity of the reference rate used, such as three months or six months LIBOR. + References No definition available. + Details
Name: | at_DebtInstrumentMinimumVariableRateBasis
Namespace Prefix: | at_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition The leverage ratio as defined in the Credit Agreement, which is part of the requirements in the negative covenants of the Credit Agreement. + References No definition available. + Details
Name: | at_LeverageRatioAsDefinedInCreditAgreement
Namespace Prefix: | at_
Data Type: | xbrli:pureItemType
Balance Type: | na
Period Type: | instant

X
- Definition Upsizing borrowing capacity under the credit facility. + References No definition available. + Details
Name: | at_LineOfCreditFacilityUpsizingBorrowingCapacity
Namespace Prefix: | at_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt, classified as current. Includes, but not limited to, notes payable, bonds payable, mortgage loans and commercial paper. Excludes capital lease obligations and debentures. + References No definition available. + Details
Name: | at_LongTermDebtCurrentExcludingDebentures
Namespace Prefix: | at_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, mortgage loans and commercial paper. Excludes capital lease obligations and debentures. + References No definition available. + Details
Name: | at_LongTermDebtNoncurrentExcludingConvertibleDebentures
Namespace Prefix: | at_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The amount of debt that was repriced on the effective date. + References No definition available. + Details
Name: | at_LongTermDebtRepricedAmount
Namespace Prefix: | at_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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=109500613&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=119400593&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=6877327&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=6876686&loc=d3e534808-122878 + Details
Name: | us-gaap_DebtInstrumentCarryingAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Contractual interest rate for funds borrowed, under the debt agreement. + 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 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_DebtInstrumentInterestRateStatedPercentage
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_DebtInstrumentLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount, after accumulated amortization, of debt discount. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 1A -URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28541-108399 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_DebtInstrumentUnamortizedDiscount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement. + 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(22)) -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 -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 Reference 3: 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=119400593&loc=d3e572229-122910 + Details
Name: | us-gaap_LineOfCredit
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility. + 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 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt, classified as current. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations. + 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 210.5-02.19,20) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_LongTermDebtCurrent
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The cash outflow for loan and debt issuance costs. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3291-108585 + Details
Name: | us-gaap_PaymentsOfFinancingCosts
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset. + 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 210.5-02.17) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_UnamortizedDebtIssuanceExpense
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Write-off of amounts previously capitalized as debt issuance cost in an extinguishment of debt. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.8) -URI http://asc.fasb.org/extlink&oid=116634182&loc=SL114868664-224227 + Details
Name: | us-gaap_WriteOffOfDeferredDebtIssuanceCost
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Details
Name: | us-gaap_DebtInstrumentAxis=us-gaap_SecuredDebtMember
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=at_SeniorUnsecuredNotesDueJune2036Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=at_CadillacTermLoanDue2025Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

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

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

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

X
- Details
Name: | us-gaap_DebtInstrumentAxis=at_SeniorSecuredTermLoanFacilityDue2025Member
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: |