v2.4.0.8

Commitments and contingencies (Details) (USD $) In Millions, unless otherwise specified | 0 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 09, 2013 item | Aug. 19, 2013 item | May 07, 2013 item | Sep. 30, 2013 item | Oct. 04, 2013
Commitments and contingencies |  |  |  |  |
Number of District Court actions alleged from July 23, 2010 to March 4, 2013 |  |  |  | 3 |
Number of District Court actions alleged from August 8, 2012 to February 28, 2013 |  |  |  | 2 |
Number of actions were succeeded by specific action with an amended claim |  |  |  | 3 |
Number of actions by which specific Ontario actions were succeeded |  |  |  | 1 |
Shareholder class action lawsuits |  |  |  |  |
Commitments and contingencies |  |  |  |  |
Number of purported securities fraud class action complaints filed |  |  |  | 5 |
Number of action complaints filed ordered to consolidate |  | 5 |  |  |
Number of plaintiff applicants directed to file supplemental submissions | 2 |  |  |  |
Number of notices of actions relating to Canadian securities class action claims |  |  |  | 3 |
Estimate damages claimed for statutory misrepresentation |  |  |  |  | $ 197.4
Number of District Court actions in which discoveries are stayed |  |  |  | 5 |
Number of District Court actions alleged in specific period |  |  |  | 5 |
Number of investors filed motion |  |  | 6 |  |

X
- Definition Represents the number of actions by which specific Ontario actions were succeeded. + References No definition available. + Details
Name: | at_LossContingencyNumberOfActionsByWhichOntarioActionsSucceeded
Namespace Prefix: | at_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the number of actions were succeeded by specific action with an amended claim being issued on behalf of Jacqeline Coffin and Sandra Lowry. + References No definition available. + Details
Name: | at_LossContingencyNumberOfActionsSucceededBySpecificActionWithAnAmendedClaimBeingIssued
Namespace Prefix: | at_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the total number of claims filed pertaining to a loss contingency during the period ordered by court to consolidate. + References No definition available. + Details
Name: | at_LossContingencyNumberOfClaimsOrderedToConsolidate
Namespace Prefix: | at_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the number of District Court actions in which discoveries are stayed. + References No definition available. + Details
Name: | at_LossContingencyNumberOfDistrictCourtActionsAlleged
Namespace Prefix: | at_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the number of District Court actions alleged in the stated prior period. + References No definition available. + Details
Name: | at_LossContingencyNumberOfDistrictCourtActionsAllegedInPriorPeriod
Namespace Prefix: | at_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the number of District Court actions alleged in the specific period. + References No definition available. + Details
Name: | at_LossContingencyNumberOfDistrictCourtActionsAllegedInSpecificPeriod
Namespace Prefix: | at_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the number of District Court actions alleged in the specific prior period. + References No definition available. + Details
Name: | at_LossContingencyNumberOfDistrictCourtActionsAllegedInSpecificPriorPeriod
Namespace Prefix: | at_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the number of notices of actions relating to the specified securities class action claims, which are filed against the defendants. + References No definition available. + Details
Name: | at_LossContingencyNumberOfNoticesOfActionsRelatingToSpecifiedSecuritiesClassActionClaims
Namespace Prefix: | at_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the number of plaintiff applicants who are directed to file supplemental submissions. + References No definition available. + Details
Name: | at_LossContingencyNumberOfPlaintiffApplicantsDirectedToFileSupplementalSubmissions
Namespace Prefix: | at_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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

X
- Definition Reflects the estimated amount of loss from the specified contingency as of the balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14435-108349 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 9 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14557-108349 + Details
Name: | us-gaap_LossContingencyEstimateOfPossibleLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The total number of new claims filed pertaining to a loss contingency during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14435-108349 + Details
Name: | us-gaap_LossContingencyNewClaimsFiledNumber
Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number of plaintiffs that have filed claims pertaining to a loss contingency. + References No definition available. + Details
Name: | us-gaap_LossContingencyNumberOfPlaintiffs
Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration