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Operations and Significant Accounting Policies - Reclassification of Prior Income Statement (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2016
Operating Revenue to Operating Revenue – Utility [Member]
Reclassification of Prior Income Statement [Line Items]
Prior Period Reclassification Amount	$ 253.3	$ 740.5
Operating Revenue to Operating Revenue – Non-utility [Member]
Reclassification of Prior Income Statement [Line Items]
Prior Period Reclassification Amount	96.3	257.7
Cost of Sales to Fuel, Purchased Power and Gas – Utility [Member]
Reclassification of Prior Income Statement [Line Items]
Prior Period Reclassification Amount	$ 0.7	$ 4.6

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

Name:	ale_ScheduleofPriorPeriodReclassificationAdjustmentsLineItems
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

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

X

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_IncomeStatementLocationAxis=ale_OperatingRevenueNonutilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_IncomeStatementLocationAxis=ale_FuelPurchasedPowerandGasUtilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: