Name:	at_DerivativeNotionalAmountThroughEndOfYearTwo
Namespace Prefix:	at_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Represents the monetary amount of purchases through the year four specified by the derivative(s). Expressed as an absolute value.

No definition available.

Name:	at_DerivativeNotionalAmountThroughYearFour
Namespace Prefix:	at_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

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

No definition available.

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

Represents the percentage of the hedged expected dividend and convertible debenture interest.

No definition available.

Name:	at_DerivativePercentageOfExpectedDividendAndConvertibleDebentureInterestPaymentsHedge
Namespace Prefix:	at_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Represents the percentage of natural gas purchased at spot market price after expiration of the fuel supply agreement until the termination of PPA.

No definition available.

Name:	at_DerivativePercentageOfNaturalGasPurchasedAtSpotMarketPriceAfterExpirationOfFuelSupplyAgreement
Namespace Prefix:	at_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Represents the remaining percentage of the entity's share of the expected natural gas purchase which was purchased at spot market price.

No definition available.

Name:	at_DerivativeRemainingPercentageOfNaturalGasPurchasedAtSpotMarketPrice
Namespace Prefix:	at_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

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.

No definition available.

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

Fixed interest rate that will be received or paid upon the exercise of the interest rate swaption contract after the amendment in the swap agreement.

No definition available.

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

Aggregate notional amount of derivative expressed in nonmonetary units. For example, the number of barrels specified in a fuel oil forward purchase contract.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 12

Name:	invest_DerivativeNonmonetaryNotionalAmount
Namespace Prefix:	invest_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

The percentage points added to the reference rate to compute the variable rate on the interest rate derivative.

No definition available.

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

The contractual rate at which a foreign currency can be purchased or sold under the terms of a foreign currency derivative contract.

No definition available.

Name:	us-gaap_DerivativeForwardExchangeRate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

High end of the range of percentage points added to the reference rate to compute the variable rate on the group of interest rate derivatives.

No definition available.

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

- Details

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

Low end of the range of percentage points added to the reference rate to compute the variable rate on the group of interest rate derivatives.

No definition available.

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

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

No definition available.

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

The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 159

-Paragraph 18

-Subparagraph f

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher AICPA

-Name Accounting Principles Board Opinion (APB)

-Number 18

-Paragraph 20

-Subparagraph a (1)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 323

-SubTopic 10

-Section 50

-Paragraph 3

-Subparagraph (a)(1)

-URI http://asc.fasb.org/extlink&oid=6382943&loc=d3e33918-111571

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

Aggregate notional amount of all derivatives. The notional amount relates to a number of currency units, shares, bushels, pounds, or other units specified in a derivative instrument.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Notional Amount

-URI http://asc.fasb.org/extlink&oid=6519104