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Investment in ATC (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 0 Months Ended
--- | --- | ---
Mar. 31, 2015 | Mar. 31, 2014 | Apr. 30, 2015
Investment in ATC [Abstract] |  |  |
Ownership Percentage | 8.00%us-gaap_EquityMethodInvestmentOwnershipPercentage |  |
Approved Return on Common Equity | 12.20%ale_EquityMethodInvestmentApprovedReturnOnCommonEquity |  |
Equity Method Investment, Expected Additional Investment | $ 1.1 ale_EquityMethodInvestmentExpectedAdditionalInvestment |  |
Subsequent Event [Line Items] |  |  |
Cash Investments | 0.4 us-gaap_PaymentsToAcquireEquityMethodInvestments |  |
ALLETE's Investment in ATC [Roll Forward] |  |  |
Equity Investment Balance as of December 31, 2014 | 121.1 us-gaap_EquityMethodInvestments |  |
Cash Investments | 0.4 us-gaap_PaymentsToAcquireEquityMethodInvestments |  |
Equity Earnings in ATC | 3.9 us-gaap_IncomeLossFromEquityMethodInvestments | 5.1 us-gaap_IncomeLossFromEquityMethodInvestments |
Distributed ATC Earnings | (3.1)us-gaap_EquityMethodInvestmentDividendsOrDistributions |  |
Equity Investment Balance as of March 31, 2015 | 122.3 us-gaap_EquityMethodInvestments |  |
ATC Summarized Financial Data [Abstract] |  |  |
Revenue | 152.4 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationRevenue | 163.3 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationRevenue |
Operating Expense | 80.0 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationCostOfSales | 78.6 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationCostOfSales |
Other Expense | 24.4 ale_EquityMethodInvestmentSummarizedFinancialInformationOtherExpense | 21.6 ale_EquityMethodInvestmentSummarizedFinancialInformationOtherExpense |
Net Income | 48.0 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationNetIncomeLoss | 63.1 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationNetIncomeLoss |
ALLETE's Equity in Net Income | 3.9 us-gaap_IncomeLossFromEquityMethodInvestments | 5.1 us-gaap_IncomeLossFromEquityMethodInvestments |
Subsequent Event [Member] |  |  |
Subsequent Event [Line Items] |  |  |
Cash Investments |  |  | 0.4 us-gaap_PaymentsToAcquireEquityMethodInvestments / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember
ALLETE's Investment in ATC [Roll Forward] |  |  |
Cash Investments |  |  | $ 0.4 us-gaap_PaymentsToAcquireEquityMethodInvestments / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember

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