Operations and Significant Accounting Policies - New Accounting Standards (Details)	Jun. 30, 2015
New Accounting Standards [Abstract]
Percentage of Long-Term Debt, Issuance Costs	1.00%

Represents the percentage of debt issuance costs in long-term debt

No definition available.

Name:	ale_PercentageofLongTermDebtIssuanceCosts
Namespace Prefix:	ale_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant