v3.19.3.a.u2

Equity method investments in unconsolidated affiliates - impairments (Details) $ in Millions | 3 Months Ended | 12 Months Ended | 41 Months Ended |
--- | --- | --- | --- | ---
Dec. 31, 2017 USD ($) agreement | Jun. 30, 2017 USD ($) | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2017 USD ($) | Jun. 18, 2018
Equity in earnings of unconsolidated affiliates |  |  |  |  |  |  |
Percentage of equity method ownership interest |  |  |  |  |  |  | 100.00%
Equity in earnings of unconsolidated affiliates |  |  | $ (3.0) | $ 43.2 | $ (54.8) |  |
Equity Method Investments. |  |  | 96.6 | 140.8 |  |  |
Distributions from unconsolidated affiliates |  |  | 59.5 | 61.6 | 47.3 |  |
Cumulative project loss |  |  | $ (62.5) | (18.4) | (102.1) |  |
Frederickson |  |  |  |  |  |  |
Equity in earnings of unconsolidated affiliates |  |  |  |  |  |  |
Percentage of equity method ownership interest |  |  | 50.20% |  |  |  |
Equity in earnings of unconsolidated affiliates |  |  | $ 9.1 | 6.9 | (27.9) |  |
Number of PPAs under which the project operates | agreement | 3 |  |  |  |  |  |
Equity Method Investments. | $ 108.3 |  | $ 65.2 | 72.0 | 108.3 | $ 108.3 |
Chambers Cogen, LP |  |  |  |  |  |  |
Equity in earnings of unconsolidated affiliates |  |  |  |  |  |  |
Percentage of equity method ownership interest |  |  | 40.00% |  |  |  |
Equity in earnings of unconsolidated affiliates |  |  | $ (46.0) | 5.6 | (42.6) |  |
Equity Method Investments. |  | $ 124.0 | $ 9.0 | $ 64.3 |  |  |
Selkirk Cogen Partners, LP |  |  |  |  |  |  |
Equity in earnings of unconsolidated affiliates |  |  |  |  |  |  |
Equity in earnings of unconsolidated affiliates |  |  |  |  | (10.6) |  |
Distributions from unconsolidated affiliates |  |  |  |  |  | 0.0 |
Cumulative project loss |  |  |  |  |  | $ (2.6) |
Earnings from unconsolidated affiliates | Frederickson |  |  |  |  |  |  |
Equity in earnings of unconsolidated affiliates |  |  |  |  |  |  |
Equity impairment other than temporary |  |  |  |  | 28.3 |  |
Earnings from unconsolidated affiliates | Chambers Cogen, LP |  |  |  |  |  |  |
Equity in earnings of unconsolidated affiliates |  |  |  |  |  |  |
Percentage of equity method ownership interest |  |  | 40.00% |  |  |  |
Equity impairment other than temporary |  | 47.1 | $ 49.2 |  | 47.1 |  |
Equity Method Investments. |  |  | $ 58.2 |  |  |  |
Earnings from unconsolidated affiliates | Selkirk Cogen Partners, LP |  |  |  |  |  |  |
Equity in earnings of unconsolidated affiliates |  |  |  |  |  |  |
Equity impairment other than temporary |  | $ 10.6 |  |  | 10.6 |  |
Gain on sale of equity method investment |  |  |  |  | $ 1.0 |  |

X
- Definition The number of power purchase agreements under which the project operates. + References No definition available. + Details
Name: | at_NumberOfPowerPurchaseAgreementsProjectOperatesUnder
Namespace Prefix: | at_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of distribution received from equity method investee for return on investment, classified as operating activities. Excludes distribution for return of investment, classified as investing activities. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3602-108585 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 220 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04(10)) -URI http://asc.fasb.org/extlink&oid=116637232&loc=SL114874131-224263 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03(12)) -URI http://asc.fasb.org/extlink&oid=116634182&loc=SL114868664-224227 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 21D -URI http://asc.fasb.org/extlink&oid=118932676&loc=SL94080555-108585 + Details
Name: | us-gaap_EquityMethodInvestmentDividendsOrDistributions
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition This item represents an other than temporary decline in value that has been recognized against an investment accounted for under the equity method of accounting. The excess of the carrying amount over the fair value of the investment represents the amount of the write down which is or was reflected in earnings. The written down value is a new cost basis with the adjusted value of the investment becoming its new carrying value subject to the equity accounting method. Evidence of a loss in value might include, but would not necessarily be limited to, absence of an ability to recover the carrying amount of the investment or inability of the investee to sustain an earnings capacity which would justify the carrying amount of the investment. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 35 -Paragraph 32 -URI http://asc.fasb.org/extlink&oid=118954474&loc=d3e32787-111569 + Details
Name: | us-gaap_EquityMethodInvestmentOtherThanTemporaryImpairment
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=114001798&loc=d3e33918-111571 + Details
Name: | us-gaap_EquityMethodInvestmentOwnershipPercentage
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Amount of gain (loss) on sale or disposal of an equity method investment. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03(b)(7)(c)) -URI http://asc.fasb.org/extlink&oid=116634182&loc=SL114868664-224227 Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03(b)(9)(a)) -URI http://asc.fasb.org/extlink&oid=116634182&loc=SL114868664-224227 + Details
Name: | us-gaap_EquityMethodInvestmentRealizedGainLossOnDisposal
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition This item represents the carrying amount on the entity's balance sheet of its investment in common stock of an equity method investee. This is not an indicator of the fair value of the investment, rather it is the initial cost adjusted for the entity's share of earnings and losses of the investee, adjusted for any distributions (dividends) and other than temporary impairment (OTTI) losses recognized. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=109237563&loc=d3e33749-111570 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 280 -SubTopic 10 -Section 50 -Paragraph 25 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8813-108599 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.12) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_EquityMethodInvestments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of income (loss) for proportionate share of equity method investee's income (loss). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03(12)) -URI http://asc.fasb.org/extlink&oid=116634182&loc=SL114868664-224227 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 220 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04(10)) -URI http://asc.fasb.org/extlink&oid=116637232&loc=SL114874131-224263 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3602-108585 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 220 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04(13)(f)) -URI http://asc.fasb.org/extlink&oid=116637391&loc=SL114874048-224260 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 280 -SubTopic 10 -Section 50 -Paragraph 22 -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8736-108599 Reference 6: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=109237563&loc=d3e33749-111570 + Details
Name: | us-gaap_IncomeLossFromEquityMethodInvestments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition This element represents the undistributed income (or loss) of equity method investments, net of dividends or other distributions received from unconsolidated subsidiaries, certain corporate joint ventures, and certain noncontrolled corporations; such investments are accounted for under the equity method of accounting. This element excludes distributions that constitute a return of investment, which are classified as investing activities. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3602-108585 + Details
Name: | us-gaap_IncomeLossFromEquityMethodInvestmentsNetOfDividendsOrDistributions
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- References No definition available. + Details
Name: | us-gaap_IncomeLossFromEquityMethodInvestmentsNetOfDividendsOrDistributionsAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=at_FredericksonMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=at_ChambersCogenLPMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=at_SelkirkCogenPartnersLPMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_IncomeStatementLocationAxis=at_IncomeLossFromEquityMethodInvestmentsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |