Regulatory Matters - Boswell Mercury Emissions Reduction Plan (Details) (Boswell Unit 4 [Member], USD $) In Millions, unless otherwise specified	Mar. 31, 2015
Boswell Unit 4 [Member]
Schedule of Utility Generating Facilities [Line Items]
Estimated Capital Expenditures	$ 260
Capital Cost Spent to Date	$ 162

The estimated amount of certain capital expenditures related to environmental compliance, regulatory matters, or other commitments and/or contingencies. This estimate includes expenditures expected in future periods and expenditures made through the end of the current period.

No definition available.

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