XML 55 R42.htm IDEA: XBRL DOCUMENT
v3.10.0.1

Operations and Significant Accounting Policies - Reclassification of Prior Income Statement (Details) - Accounting Standards Update 2017-07 [Member] - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2017 | Sep. 30, 2017
Operating and Maintenance [Member] |  |
Reclassification of Prior Income Statement [Line Items] |  |
Prior Period Reclassification Amount | $ 1.1 | $ 3.2
Cost of Sales – Non-utility [Member] |  |
Reclassification of Prior Income Statement [Line Items] |  |
Prior Period Reclassification Amount | 0.1 | 0.2
Other Income (Expense) [Member] |  |
Reclassification of Prior Income Statement [Line Items] |  |
Prior Period Reclassification Amount | $ 1.0 | $ 3.0

X
- Definition [Line Items] for Schedule of prior period reclassification adjustments to conform the presentation of prior period financial statements with the presentation in the current period. + References No definition available. + Details
Name: | ale_ScheduleofPriorPeriodReclassificationAdjustmentsLineItems
Namespace Prefix: | ale_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition The amount of a reclassification adjustment made to prior period financial statement amounts. + References No definition available. + Details
Name: | us-gaap_PriorPeriodReclassificationAdjustment
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Details
Name: | us-gaap_AdjustmentsForNewAccountingPronouncementsAxis=us-gaap_AccountingStandardsUpdate201707Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_IncomeStatementLocationAxis=ale_OperatingandMaintenanceMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_IncomeStatementLocationAxis=ale_CostofSalesNonutilityMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_IncomeStatementLocationAxis=ale_OtherIncomeExpenseMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |