v3.19.3.a.u2

Preferred shares issued by a subsidiary company - (Details) $ in Millions |  | 2 Months Ended | 12 Months Ended
--- | --- | --- | ---
Dec. 31, 2019 $ / shares shares | Feb. 27, 2020 USD ($) shares | Dec. 31, 2019 USD ($) shares | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2014 | Dec. 31, 2009 $ / shares shares | Dec. 31, 2007 $ / shares shares
Preferred shares issued by a subsidiary company |  |  |  |  |  |  |  |
Preferred shares repurchased and retired | $ |  |  | $ 8.0 | $ 8.0 | $ 3.1 |  |  |
Net income attributable to preferred shares of a subsidiary company | $ |  |  | 1.2 | (0.4) | (5.6) |  |  |
Cumulative Preferred Stock |  |  |  |  |  |  |  |
Preferred shares issued by a subsidiary company |  |  |  |  |  |  |  |
Preferred shares repurchased and retired | $ |  |  | $ 8.0 | 8.0 | 3.1 |  |  |
Series 1 Shares |  |  |  |  |  |  |  |
Preferred shares issued by a subsidiary company |  |  |  |  |  |  |  |
Preferred shares repurchased and retired (in shares) |  |  | 427,500 |  |  |  |  |
Series 1 Shares | Subsequent event |  |  |  |  |  |  |  |
Preferred shares issued by a subsidiary company |  |  |  |  |  |  |  |
Preferred shares repurchased and retired (in shares) |  | 247,894 |  |  |  |  |  |
Preferred shares repurchased and retired | $ |  | $ 3.1 |  |  |  |  |  |
Series 2 Shares |  |  |  |  |  |  |  |
Preferred shares issued by a subsidiary company |  |  |  |  |  |  |  |
Preferred shares repurchased and retired (in shares) |  |  | 100,377 |  |  |  |  |
Series 3 Shares |  |  |  |  |  |  |  |
Preferred shares issued by a subsidiary company |  |  |  |  |  |  |  |
Preferred shares repurchased and retired (in shares) |  |  | 148,311 |  |  |  |  |
Subsidiary Issuer | Cumulative Preferred Stock |  |  |  |  |  |  |  |
Preferred shares issued by a subsidiary company |  |  |  |  |  |  |  |
Dividend paid | $ |  |  | $ 7.4 | 8.3 | 8.7 |  |  |
Net income attributable to preferred shares of a subsidiary company | $ |  |  | $ 8.6 | $ 7.9 | $ 3.0 |  |  |
Subsidiary Issuer | Series 1 Shares |  |  |  |  |  |  |  |
Preferred shares issued by a subsidiary company |  |  |  |  |  |  |  |
Number of shares issued in transaction |  |  |  |  |  |  |  | 5,000,000
Dividend rate on preferred shares (as a percent) |  |  |  |  |  |  |  | 4.85%
Issuance price per share | $ / shares |  |  |  |  |  |  |  | $ 25.00
Preferred Stock, Shares Outstanding | 3,847,500 |  |  |  |  |  |  |
Redemption price after specified date and thereafter (in Canadian dollars per share) | $ / shares |  |  |  |  |  |  |  | $ 25.00
Subsidiary Issuer | Series 2 Shares |  |  |  |  |  |  |  |
Preferred shares issued by a subsidiary company |  |  |  |  |  |  |  |
Number of shares issued in transaction |  |  |  |  |  |  | 4,000,000 |
Dividend rate on preferred shares (as a percent) | 5.67% |  |  |  |  |  | 7.00% |
Issuance price per share | $ / shares |  |  |  |  |  |  | $ 25.00 |
Preferred Stock, Shares Outstanding | 2,504,131 |  |  |  |  |  |  |
Redemption price after specified date and thereafter (in Canadian dollars per share) | $ / shares | $ 25.00 |  |  |  |  |  |  |
Period for declaration of dividend at reset rate |  |  |  |  |  | 5 years |  |
Reference rate for dividend |  |  |  |  |  | five-year Government of Canada bond yield |  |
Percentage points added to the reference rate |  |  |  |  |  | 4.18% |  |
Shares Converted |  |  | 23,618 |  |  |  |  |
Subsidiary Issuer | Series 3 Shares |  |  |  |  |  |  |  |
Preferred shares issued by a subsidiary company |  |  |  |  |  |  |  |
Dividend rate on preferred shares (as a percent) | 5.83% |  | 5.83% |  |  |  |  |
Preferred Stock, Shares Outstanding | 1,077,391 |  |  |  |  |  |  |
Redemption price after specified date and thereafter (in Canadian dollars per share) | $ / shares | $ 25.00 |  |  |  |  |  |  |
Period for declaration of dividend at reset rate |  |  | 5 years |  |  |  |  |
Reference rate for dividend |  |  | 90-day Government of Canada Treasury bill rate |  |  |  |  |
Percentage points added to the reference rate | 4.18% |  |  |  |  |  |  |
Shares Converted |  |  | 295,032 |  |  |  |  |

X
- Definition Amount of net income (loss) attributable to preferred shares dividends of noncontrolling interest. + References No definition available. + Details
Name: | at_NetIncomeLossAttributableToNoncontrollingInterestInPreferredSharesOfSubsidiary
Namespace Prefix: | at_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Number of preferred shares that have been repurchased and retired during the period. + References No definition available. + Details
Name: | at_PreferredSharesRepurchasedAndRetiredDuringPeriodShares
Namespace Prefix: | at_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

X
- Definition Equity impact of the value of preferred shares that have been repurchased and retired during the period. The excess of the purchase price over par value can be charged against retained earnings (once the excess is fully allocated to additional paid in capital). + References No definition available. + Details
Name: | at_PreferredSharesRepurchasedAndRetiredDuringPeriodValue
Namespace Prefix: | at_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- 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

X
- 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

X
- 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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4313-108586 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4304-108586 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4332-108586 + Details
Name: | us-gaap_ConversionOfStockSharesConverted1
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of cash outflow in the form of ordinary dividends to preferred shareholders of the parent entity. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 15 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3291-108585 + Details
Name: | us-gaap_PaymentsOfDividendsPreferredStockAndPreferenceStock
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Description of basis of dividend, such as adjustable rate. Excludes percentage rate dividend payments or fixed dollar amounts per share. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(27)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21475-112644 + Details
Name: | us-gaap_PreferredStockDividendPaymentRateVariable
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- 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

X
- 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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21488-112644 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21475-112644 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21564-112644 + Details
Name: | us-gaap_PreferredStockRedemptionPricePerShare
Namespace Prefix: | us-gaap_
Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | instant

X
- Definition Aggregate share number for all nonredeemable preferred stock (or preferred stock redeemable solely at the option of the issuer) held by stockholders. Does not include preferred shares that have been repurchased. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -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 + Details
Name: | us-gaap_PreferredStockSharesOutstanding
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

X
- Definition The number of shares issued or sold by the subsidiary or equity method investee per stock transaction. + References No definition available. + Details
Name: | us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

X
- Definition Per share amount received by subsidiary or equity investee for each share of common stock issued or sold in the stock transaction. + References No definition available. + Details
Name: | us-gaap_SaleOfStockPricePerShare
Namespace Prefix: | us-gaap_
Data Type: | num:perShareItemType
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
Name: | us-gaap_SubsidiarySaleOfStockLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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

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

X
- Details
Name: | us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

X
- Details
Name: | us-gaap_StatementClassOfStockAxis=at_CumulativeFloatingRatePreferredSharesSeries3Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_ConsolidatedEntitiesAxis=srt_SubsidiaryIssuerMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |