v2.4.0.8

Equity method investments (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2013 | Jun. 30, 2012 | Jun. 30, 2013 | Jun. 30, 2012
Operating results |  |  |  |
Project revenue | $ 54.6 | $ 53.5 | $ 107.2 | $ 106.9
Project expenses | 45.6 | 45.3 | 88.8 | 90.8
Project other expenses | (0.3) | (2.7) | (2.5) | (7.7)
Project income | 8.7 | 5.5 | 15.9 | 8.4
Chambers |  |  |  |
Operating results |  |  |  |
Project revenue | 13.4 | 14.7 | 26.6 | 28.0
Project expenses | 11.1 | 8.7 | 20.7 | 18.5
Project other expenses | (0.6) | (0.4) | (1.2) | (1.6)
Project income | 1.7 | 5.6 | 4.7 | 7.9
Other |  |  |  |
Operating results |  |  |  |
Project revenue | 41.2 | 38.8 | 80.6 | 78.9
Project expenses | 34.5 | 36.6 | 68.1 | 72.3
Project other expenses | 0.3 | (2.3) | (1.3) | (6.1)
Project income | $ 7.0 | $ (0.1) | $ 11.2 | $ 0.5

X
- Definition The aggregate amount of income (expense) from major activities considered as part of the normal operations of the business reported by an equity method investment of the entity. + References No definition available. + Details
Name: | at_EquityMethodInvestmentSummarizedFinancialInformationProjectIncomeExpenses
Namespace Prefix: | at_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The amount of the cost of sales reported by an equity method investment of the entity. + References No definition available. + Details
Name: | us-gaap_EquityMethodInvestmentSummarizedFinancialInformationCostOfSales
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Details
Name: | us-gaap_EquityMethodInvestmentSummarizedFinancialInformationIncomeStatementAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition The amount of net income (loss) reported by an equity method investment of the entity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 28 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=6957238&loc=d3e14064-108612 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6382943&loc=d3e33918-111571 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(g)) -URI http://asc.fasb.org/extlink&oid=6881521&loc=d3e23780-122690 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6382943&loc=d3e33912-111571 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 08 -Paragraph g -Subparagraph 1, 2 -Article 4 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph w -Article 1 Reference 7: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Accounting Principles Board Opinion (APB) -Number 18 -Paragraph 20 -Subparagraph d -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 8: 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 9: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph bb -Article 1 + Details
Name: | us-gaap_EquityMethodInvestmentSummarizedFinancialInformationNetIncomeLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The amount of revenue from sale of goods and services reduced by sales returns, allowances, and discounts reported by an equity method investment of the entity. + References No definition available. + Details
Name: | us-gaap_EquityMethodInvestmentSummarizedFinancialInformationRevenue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration