v2.4.0.8

Derivative instruments and hedging activities (Details) In Millions, unless otherwise specified	9 Months Ended				9 Months Ended			9 Months Ended								9 Months Ended		1 Months Ended	9 Months Ended
	Sep. 30, 2013 Swaption interest rate from December 31, 2012 to December 31, 2024	Sep. 30, 2013 Swaption interest rate from December 2024 to December 2030	Sep. 30, 2013 Natural gas swaps MMBTU	Dec. 31, 2012 Natural gas swaps MMBTU	Sep. 30, 2013 Natural gas swaps during 2014 and 2015 Orlando project MMBTU	Sep. 30, 2013 Natural gas swaps during 2016 and 2017 Orlando project MMBTU	Sep. 30, 2013 Interest rate swaps Epsilon	Sep. 30, 2013 Interest rate swaps Rockland Wind Farm item	Sep. 30, 2013 Interest rate swaps Swaption interest rate until February 15, 2015	Sep. 30, 2013 Interest rate swaps Swaption interest rate from February 16, 2015 to February 15, 2019	Sep. 30, 2013 Interest rate swaps Swaption interest rate from February 16, 2019 to February 15, 2023	Sep. 30, 2013 Interest rate swaps Swaption interest rate after February 15, 2023	Sep. 30, 2013 Interest rate swaps Swaption interest rate until February 29, 2016 Piedmont	Sep. 30, 2013 Interest rate swaps Swaption interest rate from February, 2016 to November, 2017 Piedmont	Sep. 30, 2013 Interest rate swaps Swaption interest rate from November, 2017 to November, 2030 Piedmont	Sep. 30, 2013 Interest rate swaps Swaption interest rate from December 30, 2011 to December 31, 2026 Rockland Wind Farm	Sep. 30, 2013 Interest rate swaps Swaption interest rate from December 31, 2026 to December 31, 2031 Rockland Wind Farm	Apr. 30, 2013 Foreign currency forward contracts USD ($)	Sep. 30, 2013 Foreign currency forward contracts CAD
Derivative instruments
Percentage of ownership interest			50.00%
Notional amount (in Mmbtu)			5,600,000	10,600,000	3,200,000	1,300,000
Percentage of the entity's share in required natural gas purchases hedge					74.00%	38.00%
Swaption interest rate (as a percent)							7.40%		6.00%	6.10%	6.30%	6.40%	1.70%	4.50%	4.50%
Swap percentage	2.30%	7.20%						100.00%								4.20%	7.80%
Number of tranches								2
Applicable margin, low end of range (as a percent)	2.80%												3.50%			2.30%
Applicable margin, high end of range (as a percent)	3.30%												3.80%			2.80%
Percentage of floating rate debt converted to a fixed interest rate	75.00%
Applicable margin (as a percent)														4.00%
Swaption interest rate after addition of applicable margin (as a percent)														8.50%
Percentage of expected dividend, long-term debt and convertible debenture interest payments hedge																			71.00%
Notional amount through December 2015																			34.9
Exchange rate through December 2015																			1.108
Proceeds from termination of various foreign currency forward contracts																		$ 9.4

X

- Definition

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

+ References

No definition available.

+ Details

Name:	at_DerivativeForwardExchangeRateThroughYearFour
Namespace Prefix:	at_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Represents the number of tranches into which the expected interest payments from floating LIBOR to fixed rates are structured.

+ References

No definition available.

+ Details

Name:	at_DerivativeInstrumentsNumberOfTranches
Namespace Prefix:	at_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Represents the percentage of expected interest payments swapped.

+ References

No definition available.

+ Details

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

X

- Definition

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

+ References

No definition available.

+ Details

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

X

- Definition

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

Represents the percentage of the hedged expected dividend, long-term debt and convertible debenture interest.

+ References

No definition available.

+ Details

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

X

- Definition

Represents the percentage of floating rate debt converted to a fixed interest rate.

+ References

No definition available.

+ Details

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

X

- 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

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

X

- Definition

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

+ 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

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

X

- Definition

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

+ References

No definition available.

+ Details

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

X

- Definition

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.

+ References

No definition available.

+ Details

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

X

- Details

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

X

- Definition

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.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeLowerRangeOfBasisSpreadOnVariableRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
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

- Definition

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

+ References

Reference 1: 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

+ Details

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

X

- Definition

The cash inflow provided by derivative instruments during the period, which are classified as investing activities, excluding those designated as hedging instruments.

+ References

No definition available.

+ Details

Name:	us-gaap_ProceedsFromDerivativeInstrumentInvestingActivities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration