v2.4.0.6

Accounting for derivative instruments and hedging activities (Details) (CAD) In Millions, unless otherwise specified | 12 Months Ended |  | 12 Months Ended |  |  | 12 Months Ended |  | 12 Months Ended
--- | --- | --- | --- | --- | --- | --- | --- | ---
Dec. 31, 2012 item | Dec. 31, 2012 Epsilon Power Partners | Dec. 31, 2012 Swaption interest rate from December 30, 2011 to December 31, 2026 | Dec. 31, 2012 Swaption interest rate from December 31, 2026 to December 31, 2031 | Dec. 31, 2012 Swaption interest rate from December 31, 2012 to December 31, 2024 | Dec. 31, 2012 Swaption interest rate from December 2024 to December 2030 | Dec. 31, 2012 Natural gas swaps MMBTU | Dec. 31, 2011 Natural gas swaps MMBTU | Dec. 31, 2012 Natural gas swaps during 2014 and 2015 Orlando project MMBTU | Dec. 31, 2012 Natural gas swaps during 2016 and 2017 Orlando project MMBTU | Dec. 31, 2012 Interest rate swaps Epsilon Power Partners | Dec. 31, 2012 Interest rate swaps Swaption interest rate until February 15, 2015 | Dec. 31, 2012 Interest rate swaps Swaption interest rate from February 15, 2015 to February 15, 2019 | Dec. 31, 2012 Interest rate swaps Swaption interest rate from February 16, 2019 to February 15, 2023 | Dec. 31, 2012 Interest rate swaps Swaption interest rate after February 15, 2023 | Dec. 31, 2012 Interest rate swaps Swaption interest rate until February 29, 2016 Piedmont project | Dec. 31, 2012 Interest rate swaps Swaption interest rate from February, 2016 to November, 2017 Piedmont project | Dec. 31, 2012 Interest rate swaps Swaption interest rate from November, 2017 to November, 2030 Piedmont project | Dec. 31, 2012 Foreign currency forward contracts
Derivative instruments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of ownership interest |  | 100.00% |  |  |  |  | 50.00% |  |  |  |  |  |  |  |  |  |  |  |
Percentage of the entity's share in required natural gas purchases hedge |  |  |  |  |  |  |  |  | 64.00% | 25.00% |  |  |  |  |  |  |  |  |
Swaption interest rate (as a percent) |  |  |  |  |  |  |  |  |  |  | 7.37% | 6.02% | 6.14% | 6.26% | 6.38% | 1.70% | 4.47% | 4.47% |
Swap percentage | 100.00% |  | 4.16% | 5.06% | 5.08% | 6.70% |  |  |  |  |  |  |  |  |  |  |  |  |
Number of tranches | 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Applicable margin, low end of range (as a percent) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3.50% |  |  |
Applicable margin, high end of range (as a percent) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3.75% |  |  |
Applicable margin (as a percent) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 4.00% |  |
Swaption interest rate after addition of applicable margin (as a percent) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 8.47% |  |
Percentage of expected dividend and convertible debenture interest payments hedge |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 60.00%
Notional amount through the end of 2013 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 6.0
Notional amount through December 2015 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 176.5
Exchange rate through the end of 2013 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1.134
Exchange rate through December 2015 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1.14
Notional amount (in Mmbtu) |  |  |  |  |  |  | 10,640,000 | 14,140,000 | 3,200,000 | 1,300,000 |  |  |  |  |  |  |  |  |

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 the end of year two. + References No definition available. + Details
Name: | at_DerivativeForwardExchangeRateThroughEndOfYearTwo
Namespace Prefix: | at_
Data Type: | xbrli:pureItemType
Balance Type: | na
Period Type: | instant

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 end of year two specified by the derivative(s). Expressed as an absolute value. + References No definition available. + Details
Name: | at_DerivativeNotionalAmountThroughEndOfYearTwo
Namespace Prefix: | at_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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 and convertible debenture interest. + References No definition available. + Details
Name: | at_DerivativePercentageOfExpectedDividendAndConvertibleDebentureInterestPaymentsHedge
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 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 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 Reference 3: 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. + Details
Name: | us-gaap_EquityMethodInvestmentOwnershipPercentage
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant