Guarantees and contingencies (Details)	Jun. 30, 2018 item
Guarantees and Contingencies
Number of pending matters expected to have a material adverse impact on financial position or results of operations, or have been reserved	0

Represents the number of pending matters having material adverse impact on financial position or results of operations or have been reserved.

No definition available.

Name:	at_LossContingencyPendingMattersHavingMaterialAdverseImpactOnFinancialPositionOrResultsOfOperationsOrHaveBeenReserved
Namespace Prefix:	at_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant