v3.5.0.2

Contingencies (Details) - Shareholder class action lawsuits - item |  | 6 Months Ended
--- | --- | ---
Aug. 30, 2013 | Jun. 30, 2016
Contingencies |  |
Number of notices of actions relating to Canadian securities class action claims |  | 3
Number of actions were succeeded by specific action with an amended claim | 3 |
Number of actions by which specific Ontario actions were succeeded | 1 |
Number of pending matters having material adverse impact on financial position or results of operations or have been reserved |  | 0

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 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 pending matters having material adverse impact on financial position or results of operations or have been reserved. + References No definition available. + Details
Name: | at_LossContingencyPendingMattersHavingMaterialAdverseImpactOnFinancialPositionOrResultsOfOperationsOrHaveBeenReserved
Namespace Prefix: | at_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_LossContingenciesLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_LossContingenciesByNatureOfContingencyAxis=at_ShareholderClassActionLawsuitsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |