Regulatory Matters - Conservation Improvement Program (CIP) (Details) - USD ($) $ in Millions	9 Months Ended
Sep. 30, 2015	Sep. 30, 2014
Conservation Improvement Program [Abstract]
CIP Energy-Savings Goal	1.50%
CIP Financial Incentive	$ 6.2	$ 8.7

Minnesota's Next Generation Act of 2007 introduced an energy-savings goal of 1.5 percent of net gross annual retail electric energy sales beginning with the 2010 program year.

No definition available.

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