v3.4.0.3

Convertible debentures (Details) CAD in Millions, $ in Millions		1 Months Ended	3 Months Ended	6 Months Ended	12 Months Ended
	Apr. 13, 2016 CAD	Apr. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Mar. 31, 2016 CAD	Mar. 31, 2016 USD ($)
Convertible debentures
Convertible debentures					$ 277.7		$ 271.4
Convertible Debentures
Convertible debentures
Aggregate principal amount of debt					277.7		271.4
Less: unamortized deferred financing costs					(7.7)		(6.3)
Gain on repurchase of convertible debentures					$ (3.1)
Amount that may be purchased, as a percentage of public float					10.00%
Convertible Debentures | 6.25% Debentures due March 2017
Convertible debentures
Convertible debentures					$ 48.6		$ 51.8
Aggregate principal amount of debt | CAD						CAD 67.2
Convertible debentures stated interest rate percentage						6.25%	6.25%
Convertible Debentures | 5.6% Debentures due June 2017
Convertible debentures
Convertible debentures					54.8		$ 58.3
Aggregate principal amount of debt | CAD						CAD 75.8
Convertible debentures stated interest rate percentage						5.60%	5.60%
Convertible Debentures | 5.75% Debentures due June 2019
Convertible debentures
Convertible debentures					117.0		$ 105.3
Convertible debentures stated interest rate percentage						5.75%	5.75%
Convertible Debentures | 6.00% Debenture due December 2019
Convertible debentures
Convertible debentures					65.0		$ 62.3
Aggregate principal amount of debt | CAD						CAD 81.0
Convertible debentures stated interest rate percentage						6.00%	6.00%
Convertible Debentures | Subsequent event | 6.25% Debentures due March 2017
Convertible debentures
Proceeds deposited with trustee in advance of payment of certain unsecured subordinated debentures | CAD	CAD 67.2
Convertible Debentures | Subsequent event | 5.6% Debentures due June 2017
Convertible debentures
Proceeds deposited with trustee in advance of payment of certain unsecured subordinated debentures | CAD	CAD 75.8
Convertible Debentures | Interest expense | Subsequent event | 5.6% Debentures Due June 2017 And 6.25% Debentures Due March 2017 (Member)
Convertible debentures
Write-off of deferred financing costs		$ 1.3
Convertible Debentures | NCIB
Convertible debentures
Amount that may be purchased under the NCIB					28.5
Repayment of convertible debentures				$ (18.8)	$ 24.8
Gain on repurchase of convertible debentures			$ (2.5)

X

- Definition

The maximum amount of convertible debentures that may be purchased under the Normal Course Issuer Bid.

+ References

No definition available.

+ Details

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Namespace Prefix:	at_
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Balance Type:	credit
Period Type:	duration

X

- Definition

Maximum percentage of public float that may be purchased under Normal Course Issuer Bid for the entity's convertible debentures.

+ References

No definition available.

+ Details

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X

- Definition

Repayment of debenture instrument which can be exchanged for a specified amount of another security, typically the entity's common stock, at the option of the issuer or the holder.

+ References

No definition available.

+ Details

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Balance Type:	debit
Period Type:	duration

X

- Definition

The cash proceeds deposited with the trustee for the repayment in whole of certain senior unsecured subordinated debentures.

+ References

No definition available.

+ Details

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Namespace Prefix:	at_
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X

- Definition

The portion of the carrying value of convertible subordinated debt as of the balance sheet date that is scheduled to be repaid after one year or beyond the normal operating cycle if longer. This form of debt can be exchanged for a specified amount of another security, typically the entity's common stock, at the option of the issuer or the holder, and places a lender in a lien position behind debt having a higher priority of repayment in liquidation of the entity's assets.

+ 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 210.5-02.22)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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Balance Type:	credit
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X

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=34725769&loc=d3e28878-108400

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399

+ Details

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Balance Type:	credit
Period Type:	instant

X

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ 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 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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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

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Data Type:	xbrli:stringItemType
Balance Type:	na
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X

- Definition

Represents the charge against earnings during the period for commitment fees and debt issuance expenses.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.8)
-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

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X

- Definition

Amount of gain (loss) from the difference between the repurchase price of a debt instrument initially issued by the entity and the net carrying amount of the debt at the time of its repurchase.

+ References

No definition available.

+ Details

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X

- Definition

The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

+ 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 210.5-02.17)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Details

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