v2.4.0.6

Summary of significant accounting policies (Details 2) | 12 Months Ended
--- | ---
Dec. 31, 2012 Rate
Derivative financial instruments |
Number of interest rate swaps designated as cash flow hedges | 2
Long-term incentive plan |
Long-term incentive plan |
Cliff vesting period | 3 years
Portion of vested notional units expected to be redeemed in cash | 0.33
Portion of vested notional units expected to be redeemed in shares | 0.67
Long-term incentive plan | Officers |
Long-term incentive plan |
Cliff vesting period | 3 years
Long-term incentive plan | Non-officers |
Long-term incentive plan |
Vesting period on ratable basis | 3 years
Long-term incentive plan | Senior officers |
Long-term incentive plan |
Vesting period on ratable basis | 3 years
Vesting percentage of performance units | 33.00%
Long-term incentive plan | Maximum |
Long-term incentive plan |
Vesting percentage of notional units | 150.00%
Aggregate number of shares which may be issued from treasury | 1,300,000

X
- Details
Name: | at_DerivativeFinancialInstrumentsAbstract
Namespace Prefix: | at_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the number of interest rate swaps designated as cash flow hedges for accounting purposes. + References No definition available. + Details
Name: | at_NumberOfInterestRateSwapsDesignatedAsCashFlowHedges
Namespace Prefix: | at_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition Represents the cliff vesting period of equity-based compensation awards. + References No definition available. + Details
Name: | at_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriodOnCliffBasis
Namespace Prefix: | at_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the portion of vested notional units that are expected to be redeemed in cash under equity-based compensation plan. + References No definition available. + Details
Name: | at_ShareBasedCompensationArrangementByShareBasedPaymentAwardPortionOfVestedNotionalUnitsExpectedToBeRedeemedInCash
Namespace Prefix: | at_
Data Type: | xbrli:pureItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the portion of vested notional units that are expected to be redeemed in shares of common stock under equity-based compensation plan. + References No definition available. + Details
Name: | at_ShareBasedCompensationArrangementByShareBasedPaymentAwardPortionOfVestedNotionalUnitsExpectedToBeRedeemedInShares
Namespace Prefix: | at_
Data Type: | xbrli:pureItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the vesting percentage of notional units depending on the level of achievement of target levels under equity-based compensation plan. + References No definition available. + Details
Name: | at_ShareBasedCompensationArrangementByShareBasedPaymentAwardVestingPercentageOfNotionalUnits
Namespace Prefix: | at_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the vesting percentage of performance units depending on the level of achievement of target levels under the equity-based compensation plan. + References No definition available. + Details
Name: | at_ShareBasedCompensationArrangementByShareBasedPaymentAwardVestingPercentageOfPerformanceUnits
Namespace Prefix: | at_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1
Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph A240 -Subparagraph a -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 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant