Commitments, Guarantees and Contingencies - Transmission (Details) - Great Northern Transmission Line [Member] - Transmission Line [Member] $ in Millions	Sep. 30, 2016 USD ($) kV Miles
Transmission [Line Items]
Transmission Line Length (Miles) \| Miles	220
Transmission Line Capacity (kV) \| kV	500
Minimum [Member]
Transmission [Line Items]
Estimated Capital Expenditures	$ 560
Maximum [Member]
Transmission [Line Items]
Estimated Capital Expenditures	$ 710

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