Regulated Operations - Minnesota Solar Energy Standard (Details) $ in Millions	Jun. 30, 2020 USD ($) MW
Regulated Operations [Abstract]
Estimated Investment Amount \| $	$ 40
Estimated Generation from Projects Under Development \| MW	20

Estimated Generation from Projects Under Development

No definition available.

Name:	ale_EstimatedGenerationFromProjectsUnderDevelopment
Namespace Prefix:	ale_
Data Type:	num:powerItemType
Balance Type:	na
Period Type:	instant