XML 66 R50.htm IDEA: XBRL DOCUMENT
v3.19.1

Commitments, Guarantees and Contingencies - Environmental Matters (Details) - USD ($) $ in Millions | 3 Months Ended |
--- | --- | ---
Mar. 31, 2019 | Dec. 31, 2018
Coal Combustion Residuals [Member] | Minnesota Power [Member] |  |
Environmental Matters [Line Items] |  |
Expected Period for Costs of Compliance | 15 years |
Coal Combustion Residuals [Member] | Minimum [Member] | Minnesota Power [Member] |  |
Environmental Matters [Line Items] |  |
Estimated Costs of Compliance | $ 65 |
Coal Combustion Residuals [Member] | Maximum [Member] | Minnesota Power [Member] |  |
Environmental Matters [Line Items] |  |
Estimated Costs of Compliance | 120 |
Manufactured Gas Plant [Member] | SWL&P [Member] | Superior, WI [Member] |  |
Environmental Matters [Line Items] |  |
Estimated Costs of Compliance (Accrued) | $ 7 | $ 7
Expected Period to Incur Remediation Costs | four years |

X
- Definition [Line Items] for Environmental Matters [Table] + References No definition available. + Details
Name: | ale_EnvironmentalMattersLineItems
Namespace Prefix: | ale_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Expected Period for Costs of Compliance + References No definition available. + Details
Name: | ale_ExpectedPeriodforCostsofCompliance
Namespace Prefix: | ale_
Data Type: | us-types:durationStringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Expected Period to Incur Remediation Costs + References No definition available. + Details
Name: | ale_ExpectedPeriodtoIncurRemediationCosts
Namespace Prefix: | ale_
Data Type: | us-types:durationStringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Total costs accrued as of the balance sheet date for environmental loss contingencies. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=82911808&loc=d3e14326-108349 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 25 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6395460&loc=d3e13647-108346 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 30 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13207-110859 + Details
Name: | us-gaap_AccrualForEnvironmentalLossContingencies
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Best estimate of the loss exposure for reasonably possible environmental contingencies at an individual site for which no accrual has been recorded. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 30 -Section 50 -Paragraph 10 -Subparagraph (c),(d)(2) -URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13237-110859 Reference 2: http://www.xbrl.org/2003/role/recommendedDisclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 30 -Section 50 -Paragraph 10 -Subparagraph (d)(2) -URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13237-110859 + Details
Name: | us-gaap_SiteContingencyLossExposureNotAccruedBestEstimate
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Details
Name: | us-gaap_EnvironmentalRemediationContingencyAxis=ale_CoalCombustionResidualsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

X
- Details
Name: | srt_RangeAxis=srt_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_EnvironmentalRemediationContingencyAxis=ale_ManufacturedGasPlantMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

X
- Details
Name: | srt_StatementGeographicalAxis=ale_SuperiorWIMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |