Regulatory Matters - Boswell Mercury Emissions Reduction Plan (Details) - Boswell Unit 4 [Member] $ in Millions	Sep. 30, 2015 USD ($)
Schedule of Utility Generating Facilities [Line Items]
Estimated Capital Expenditures	$ 260
Capital Expenditures to Date	$ 207

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