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Operations and Significant Accounting Policies - Income Taxes (Details) - Income Tax Expense [Member] $ in Millions	9 Months Ended
	Sep. 30, 2018 USD ($)
Measurement Period Adjustment	$ 0.9
Expected Full-year Measurement Period Adjustment	$ 1.1

X

- Definition

Expected Full-year Measurement Period Adjustment for new/clarified TCJA information

+ References

No definition available.

+ Details

Name:	ale_ExpectedFullyearMeasurementPeriodAdjustment
Namespace Prefix:	ale_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

Measurement period adjustment for new/clarified TCJA information.

+ References

No definition available.

+ Details

Name:	ale_MeasurementPeriodAdjustment
Namespace Prefix:	ale_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Details

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