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Operations and Significant Accounting Policies - Reclassification of Prior Income Statement (Details) - Accounting Standards Update 2017-07 [Member] - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2017
Operating and Maintenance [Member]
Reclassification of Prior Income Statement [Line Items]
Prior Period Reclassification Amount	$ 1.1	$ 3.2
Cost of Sales – Non-utility [Member]
Reclassification of Prior Income Statement [Line Items]
Prior Period Reclassification Amount	0.1	0.2
Other Income (Expense) [Member]
Reclassification of Prior Income Statement [Line Items]
Prior Period Reclassification Amount	$ 1.0	$ 3.0

X

- Definition

[Line Items] for Schedule of prior period reclassification adjustments to conform the presentation of prior period financial statements with the presentation in the current period.

+ References

No definition available.

+ Details

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Namespace Prefix:	ale_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

The amount of a reclassification adjustment made to prior period financial statement amounts.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Details

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Period Type:

X

- Details

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Namespace Prefix:
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Balance Type:
Period Type:

X

- Details

Name:	us-gaap_IncomeStatementLocationAxis=ale_CostofSalesNonutilityMember
Namespace Prefix:
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Balance Type:
Period Type:

X

- Details

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Namespace Prefix:
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Period Type: