Nature of business (Details)	12 Months Ended
Dec. 31, 2012 MW province state project
Nature of business
Gross generating capacity of project (in MW)	3,366
Ownership interest in operational power generation projects (in MW)	2,117
Number of operational power generation projects	29
Number of states in which power generation projects operate	11
Number of provinces in which power generation projects operate	2
Number of biomass projects under construction in Georgia	1
Generating capacity of biomass project under construction in Georgia (in MW)	53
Number of projects Which are wholly owned subsidiaries	23

Represents the generating capacity of biomass project under construction.

No definition available.

Name:	at_GeneratingCapacityOfBiomassProjectUnderConstruction
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Represents the gross power generating capacity of project.

No definition available.

Name:	at_GrossGeneratingCapacityOfProject
Namespace Prefix:	at_
Data Type:	num:powerItemType
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Period Type:	duration

Represents the number of biomass projects under construction in Georgia.

No definition available.

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

Represents the number of operational power generation projects.

No definition available.

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

Represents the number of power generation projects which are wholly-owned subsidiaries of the reporting entity.

No definition available.

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

The number of provinces in which the power generation projects operate.

No definition available.

Name:	at_NumberOfProvincesInWhichPowerGenerationProjectsOperate
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Data Type:	xbrli:integerItemType
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The number of states in which the power generation projects operate.

No definition available.

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

Represents the ownership interest in operational power generation projects.

No definition available.

Name:	at_OwnershipInterestInOperationalPowerGenerationProjects
Namespace Prefix:	at_
Data Type:	num:powerItemType
Balance Type:	na
Period Type:	duration

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Data Type:	xbrli:stringItemType
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Period Type:	duration