Represents the frequency for redemption of preferred shares after the specific date, expressed in number of years.

No definition available.

Name:	at_FrequencyAfterSpecificDateForPreferredStockRedemption
Namespace Prefix:	at_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the initial period in which the shares pay fixed cumulative dividends.

+ References

No definition available.

+ Details

Name:	at_InitialPeriodForPreferredStockFixedDividendRate
Namespace Prefix:	at_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

+ Details

Name:	at_PreferredSharesIssuedBySubsidiaryCompanyLineItems
Namespace Prefix:	at_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The percentage points added to the reference rate to compute the variable rate on the preference stock.

+ References

No definition available.

+ Details

Name:	at_PreferredStockBasisSpreadOnVariableRate
Namespace Prefix:	at_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the period in which the dividend rate will reset after the initial period has ended.

+ References

No definition available.

+ Details

Name:	at_PreferredStockDividendRateResetPeriodAfterCompletionOfInitialPeriod
Namespace Prefix:	at_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Issued price per share of nonredeemable preferred stock (or preferred stock redeemable solely at the option of the issuer).

+ References

No definition available.

+ Details

Name:	at_PreferredStockIssuedPricePerShare
Namespace Prefix:	at_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the decline in redemption price per share per year.

+ References

No definition available.

+ Details

Name:	at_PreferredStockRedemptionPricePerShareDeclinePerYear
Namespace Prefix:	at_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

The number of shares converted in a noncash (or part noncash) transaction. Noncash is defined as transactions during a period that do not result in cash receipts or cash payments in the period. "Part noncash" refers to that portion of the transaction not resulting in cash receipts or cash payments in the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4332-108586

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4304-108586

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4313-108586

+ Details

Name:	us-gaap_ConversionOfStockSharesConverted1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of cash outflow in the form of ordinary dividends to preferred shareholders of the parent entity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=56944662&loc=d3e3291-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Financing Activities
-URI http://asc.fasb.org/extlink&oid=6513228

+ Details

Name:	us-gaap_PaymentsOfDividendsPreferredStockAndPreferenceStock
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Description of basis of dividend, such as adjustable rate. Excludes percentage rate dividend payments or fixed dollar amounts per share.

+ References

+ Details

Name:	us-gaap_PreferredStockDividendPaymentRateVariable
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount per share used to calculated dividend payments on preferred stock.

+ References

No definition available.

+ Details

Name:	us-gaap_PreferredStockDividendRatePerDollarAmount
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

The percentage rate used to calculate dividend payments on preferred stock.

+ References

No definition available.

+ Details

Name:	us-gaap_PreferredStockDividendRatePercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The price per share at which the preferred stock of an entity that has priority over common stock in the distribution of dividends and in the event of liquidation of the entity is redeemed or may be called at. The redemption features of this preferred stock are solely within the control of the issuer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21475-112644

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21564-112644

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21488-112644

+ Details

Name:	us-gaap_PreferredStockRedemptionPricePerShare
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Total number of nonredeemable preferred shares (or preferred stock redeemable solely at the option of the issuer) issued to shareholders (includes related preferred shares that were issued, repurchased, and remain in the treasury). May be all or portion of the number of preferred shares authorized. Excludes preferred shares that are classified as debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.28)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 02
-Paragraph 29
-Article 5

+ Details

Name:	us-gaap_PreferredStockSharesIssued
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_StatementClassOfStockAxis=at_CumulativeRateResetPreferredShares7PercentSeries2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=us-gaap_SubsidiariesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementClassOfStockAxis=us-gaap_CumulativePreferredStockMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementClassOfStockAxis=at_CumulativePreferredShares4.85PercentSeries1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentRedemptionPeriodAxis=at_DebtInstrumentRedemptionPeriodAfterJune302016Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Preferred shares issued by a subsidiary company (Details) $ in Millions			12 Months Ended
	Jun. 30, 2012 CAD / shares	Dec. 31, 2007 CAD / shares shares	Dec. 31, 2015 CAD / shares	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($) shares	Dec. 31, 2013 USD ($)	Dec. 31, 2009 CAD / shares shares	Dec. 31, 2014 CAD / shares
Preferred shares issued by subsidiary company
Dividend paid \| $				$ 8.8	$ 14.6	$ 9.4
Series 2 Shares
Preferred shares issued by subsidiary company
Shares Converted \| shares					1,661,906
Epsilon Power Partners \| Cumulative Preferred Stock [Member]
Preferred shares issued by subsidiary company
Dividend paid \| $				$ 8.8	$ 11.6
Epsilon Power Partners \| Series 1 Shares
Preferred shares issued by subsidiary company
Number of preferred shares issued \| shares		5,000,000
Dividend rate on preferred shares (as a percent)		4.85%
Issue price of preferred shares (in Canadian dollars per share)		CAD 25.00
Dividend per share per annum (in Canadian dollars per share)			CAD 1.2125
Redemption price after specified date and thereafter (in Canadian dollars per share)	CAD 26.00
Decline in redemption price per year (in Canadian dollars per share)	0.25
Epsilon Power Partners \| Series 1 Shares \| After June 30, 2016
Preferred shares issued by subsidiary company
Redemption price after specified date and thereafter (in Canadian dollars per share)	CAD 25.00
Epsilon Power Partners \| Series 2 Shares
Preferred shares issued by subsidiary company
Number of preferred shares issued \| shares							4,000,000
Dividend rate on preferred shares (as a percent)							7.00%
Issue price of preferred shares (in Canadian dollars per share)							CAD 25.00
Dividend per share per annum (in Canadian dollars per share)							CAD 1.75
Redemption price after specified date and thereafter (in Canadian dollars per share)								CAD 25.00
Initial period for declaration of dividend							5 years
Period for declaration of dividend at reset rate							5 years
Reference rate for dividend				90-day Government of Canada treasury bill rate			Five-year Government of Canada bond yield
Percentage points added to the reference rate			4.18%	4.18%			4.18%
Frequency for redemption after December 31, 2014					5 years