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Short-Term and Long-Term Debt - Financial Covenants (Details)	Sep. 30, 2014
Financial Covenants [Abstract]
Required Indebtedness to Total Capital Ratio	0.65
Actual Indebtedness to Total Capital Ratio	0.47

X

- Definition

Actual indebtedness to total capital ratio required to be maintained by most restrictive financial covenant.

+ References

No definition available.

+ Details

Name:	ale_DebtInstrumentActualIndebtednessToTotalCapitalRatio
Namespace Prefix:	ale_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Indebtedness to total capital ratio required to be maintained by most restrictive financial covenant.

+ References

No definition available.

+ Details

Name:	ale_DebtInstrumentRequiredIndebtednessToTotalCapitalRatio
Namespace Prefix:	ale_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

X

- Details

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