Regulated Operations - Conservation Improvement Program (Details) $ in Millions	Sep. 30, 2020 USD ($)
Regulated Operations [Abstract]
Conservation Improvement Program Goal, Next Fiscal Year	$ 10.5
Conservation Improvement Program Goal, In 2 Years	10.7
Conservation Improvement Program Goal, In 3 Years	$ 10.9

Conservation Improvement Program Goal, Next Fiscal Year

No definition available.

Name:	ale_ConservationImprovementProgramGoalNextFiscalYear
Namespace Prefix:	ale_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant