XML 93 R70.htm IDEA: XBRL DOCUMENT
v3.10.0.1

Acquisitions - Acquisition of Non-Controlling Interest (Details) - USD ($) $ in Millions |  | 12 Months Ended
--- | --- | ---
Apr. 15, 2016 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Acquisition of Non-Controlling Interest [Line Items] |  |  |  |
Payments to Acquire Additional Interest in Subsidiaries |  | $ 0.0 | $ 0.0 | $ 8.0
ALLETE Clean Energy [Member] |  |  |  |
Acquisition of Non-Controlling Interest [Line Items] |  |  |  |
Payments to Acquire Additional Interest in Subsidiaries | $ 8.0 |  |  |
Gain or Loss Recognized in Net Income or Other Comprehensive Income | $ 0.0 |  |  |

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_MinorityInterestLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of gain (loss) related to nonoperating activities, classified as other. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.7,9) -URI http://asc.fasb.org/extlink&oid=115205541&loc=SL114868664-224227 + Details
Name: | us-gaap_OtherNonoperatingGainsLosses
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The cash outflow associated with the purchase of noncontrolling interest during the period. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 13 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3213-108585 + Details
Name: | us-gaap_PaymentsToAcquireAdditionalInterestInSubsidiaries
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Details
Name: | dei_LegalEntityAxis=ale_ALLETECleanEnergyMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |