Commitments, Guarantees and Contingencies - Hydro Operations (Details) (Hydro Project [Member], USD $) In Millions, unless otherwise specified	Dec. 31, 2014
Hydro Project [Member]
Hydro Operations [Line Items]
Estimated Capital Expenditures	$ 90

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