Regulatory Matters - Boswell Mercury Emissions Reduction Plan (Details) (Boswell Unit 4 [Member], USD $) In Millions, unless otherwise specified	Mar. 31, 2013
Minimum [Member]
Schedule of Utility Generating Facilities [Line Items]
Estimated Capital Expenditures	$ 350
Maximum [Member]
Schedule of Utility Generating Facilities [Line Items]
Estimated Capital Expenditures	$ 400

The estimated amount of certain future capital expenditures related to environmental compliance, regulatory matters, or other committments and/or contingencies.

No definition available.

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