v2.4.0.6

Basis of presentation and summary of significant accounting policies (Details)	3 Months Ended
	Mar. 31, 2013 MW project state province
Basis of presentation and summary of significant accounting policies
Gross generating capacity of project (in MW)	2,965
Ownership interest in operational power generation projects (in MW)	2,046
Number of operational power generation projects	28
Number of states in which power generation projects operate	10
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
Gregory
Basis of presentation and summary of significant accounting policies
Percentage of ownership interest	17.10%
Delta-Person
Basis of presentation and summary of significant accounting policies
Percentage of ownership interest	40.00%

X

- Definition

Represents the generating capacity of biomass project under construction.

+ References

No definition available.

+ Details

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

X

- Definition

Represents the gross power generating capacity of project.

+ References

No definition available.

+ Details

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Namespace Prefix:	at_
Data Type:	num:powerItemType
Balance Type:	na
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X

- Definition

Represents the number of biomass projects under construction in Georgia.

+ References

No definition available.

+ Details

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Namespace Prefix:	at_
Data Type:	xbrli:integerItemType
Balance Type:	na
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X

- Definition

Represents the number of operational power generation projects.

+ References

No definition available.

+ Details

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

X

- Definition

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

+ References

No definition available.

+ Details

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

X

- Definition

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

+ References

No definition available.

+ Details

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X

- Definition

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

+ References

No definition available.

+ Details

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

X

- Definition

Represents the ownership interest in operational power generation projects.

+ References

No definition available.

+ Details

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

X

- Definition

The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 159

-Paragraph 18

-Subparagraph f

-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 323

-SubTopic 10

-Section 50

-Paragraph 3

-Subparagraph (a)(1)

-URI http://asc.fasb.org/extlink&oid=6382943&loc=d3e33918-111571



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher AICPA

-Name Accounting Principles Board Opinion (APB)

-Number 18

-Paragraph 20

-Subparagraph a (1)

-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.

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- Details

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