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Operations and Significant Accounting Policies - Sale of U.S. Water Services (Details) $ in Millions | 12 Months Ended
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Dec. 31, 2019 USD ($)
Business Segments [Line Items] |
Proceeds from Sale of U.S. Water Services - Net of Transaction Costs and Cash Retained | $ 270.0
Gain on Sale of U.S. Water Services, After-tax | $ 13.2

X
- Definition Approximate Proceeds from Divestiture of Business + References No definition available. + Details
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- Definition Gain on Sale of Affiliate, After-tax + References No definition available. + Details
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Namespace Prefix: | ale_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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