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
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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
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Data Type: | xbrli:pureItemType
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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
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Data Type: | xbrli:integerItemType
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- Definition Represents the percentage of expected interest payments swapped. + References No definition available. + Details
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Data Type: | num:percentItemType
Balance Type: | na
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- 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
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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
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Namespace Prefix: | at_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Represents the percentage of the hedged expected dividend, long-term debt and convertible debenture interest. + References No definition available. + Details
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Namespace Prefix: | at_
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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
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- 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
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Data Type: | num:percentItemType
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- 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
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- Details
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- 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
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Data Type: | num:percentItemType
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- 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
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Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- 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
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Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- 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
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Data Type: | xbrli:monetaryItemType
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