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Regulatory Matters - Conservation Improvement Program (CIP) (Details) - USD ($) $ in Millions | 9 Months Ended
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Sep. 30, 2015 | Sep. 30, 2014
Conservation Improvement Program [Abstract] |  |
CIP Energy-Savings Goal | 1.50% |
CIP Financial Incentive | $ 6.2 | $ 8.7

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- Definition Minnesota's Next Generation Act of 2007 introduced an energy-savings goal of 1.5 percent of net gross annual retail electric energy sales beginning with the 2010 program year. + References No definition available. + Details
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Namespace Prefix: | ale_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition Conservation improvement program financial incentive approved by the public utilities commission. + References No definition available. + Details
Name: | ale_CIPFinancialIncentive
Namespace Prefix: | ale_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Conservation Improvement Program [Abstract] + References No definition available. + Details
Name: | ale_ConservationImprovementProgramAbstract
Namespace Prefix: | ale_
Data Type: | xbrli:stringItemType
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Period Type: | duration