Short-Term and Long-Term Debt - Financial Covenants (Details)	Dec. 31, 2018
Financial Covenants [Abstract]
Allowed Indebtedness to Total Capitalization Ratio	0.65
Actual Indebtedness to Total Capitalization Ratio	0.41

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

No definition available.

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