Name:	at_MinimumMarketPriceOfWeightedAverageCanadianDollarsClosingPriceRequiredToVestRemainingPortionOfTransitionNotionalShares
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Represents the minimum percentage by which weighted average Canadian dollar price of common share exceeds the market price of common share.

No definition available.

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Represents the number of consecutive calendar months when weighted average Canadian dollar closing price of common shares must exceed market price per common share by at least 50% before the 50% portion of the transition equity participation agreement may vest.

No definition available.

Name:	at_NumberOfConsecutiveMonthsWeightedAverageCanadianDollarClosingPriceExceedsMarketPriceOfCommonShareByAtLeastFiftyPercent
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The portion of notional units that were settled in cash prior to the Plan modification for non-officer employees.

No definition available.

Name:	at_PortionOfNotionalUnitsSettledInCashPriorToPlanModification
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The portion of notional units that were settled in common shares prior to the Plan modification for non-officer employees.

No definition available.

Name:	at_PortionOfNotionalUnitsSettledInCommonSharePriorToPlanModification
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The fair value of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

No definition available.

Name:	at_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedFairValue
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Data Type:	xbrli:monetaryItemType
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The number of notional shares outstanding under the Transition Equity Participation Agreement, not yet vested.

No definition available.

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Data Type:	xbrli:sharesItemType
Balance Type:	na
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The number of equity-based payment instruments, excluding stock (or unit) options that vested and redeemed during the reporting period.

No definition available.

Name:	at_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedAndRedeemedDuringPeriod
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Data Type:	xbrli:sharesItemType
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The weighted average fair value as of grant date pertaining to an equity-based plan other than a stock (or unit) option plan which became vested and were redeemed, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

No definition available.

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Data Type:	num:perShareItemType
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- References

No definition available.

Name:	at_ShareBasedCompensationArrangementsByShareBasedPaymentAwardEquityInstrumentsTransitionAgreementDisclosuresAbstract
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Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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-Topic 718
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Aggregate cash paid by the entity during the period to settle equity instruments granted under equity-based payment arrangements.

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Amount of cost to be recognized for nonvested award under share-based payment arrangement. Excludes share and unit options.

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Balance Type:	debit
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- References

No definition available.

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdditionalDisclosuresAbstract
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The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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Data Type:	xbrli:sharesItemType
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Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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-Publisher FASB
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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

No definition available.

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Per share or unit weighted-average fair value of nonvested award under share-based payment arrangement. Excludes share and unit options.

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-Publisher FASB
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Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
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Data Type:	num:perShareItemType
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Weighted average remaining contractual term for equity-based awards excluding options, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingWeightedAverageRemainingContractualTerms
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Data Type:	xbrli:durationItemType
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Price of a single share of a number of saleable stocks of a company.

No definition available.