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Commitments, Guarantees and Contingencies - Coal Rail and Shipping Contracts (Details) (Coal and Transportation [Member], USD $) In Millions, unless otherwise specified | Dec. 31, 2014
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Coal and Transportation [Member] |
Coal Rail and Shipping Contracts [Line Items] |
Coal Supply and Transportation Agreements, Minimum Annual Payment Obligation in 2015 | $ 38.3 us-gaap_RecordedUnconditionalPurchaseObligationDueWithinOneYear / us-gaap_RecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis = ale_CoalAndTransportationMember

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- Definition Amount of the recorded unconditional purchase obligation maturing in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 440 -SubTopic 10 -Section 50 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6394976&loc=d3e25383-109308 + Details
Name: | us-gaap_RecordedUnconditionalPurchaseObligationDueWithinOneYear
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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