XML 103 R80.htm IDEA: XBRL DOCUMENT
v3.10.0.1

Short-Term and Long-Term Debt - Schedule of Long-Term Debt (Details) - USD ($) $ in Millions | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2018 | Apr. 16, 2018 | Dec. 31, 2017
Debt Instrument [Line Items] |  |  |
Long-term Debt | $ 1,495.2 |  | $ 1,513.3
Unamortized Debt Issuance Costs | (9.2) |  | (10.0)
Total Long-Term Debt | 1,486.0 |  | 1,503.3
Less: Due Within One Year | 57.5 |  | 64.1
Net Long-Term Debt | $ 1,428.5 |  | 1,439.2
First Mortgage Bonds - 1.83% Series Due 2018 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate | 1.83% |  |
Long-term Debt | $ 0.0 |  | 50.0
First Mortgage Bonds - 8.17% Series Due 2019 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate | 8.17% |  |
Long-term Debt | $ 42.0 |  | 42.0
First Mortgage Bonds - 5.28% Series Due 2020 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate | 5.28% |  |
Long-term Debt | $ 35.0 |  | 35.0
First Mortgage Bonds - 2.80% Series Due 2020 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate | 2.80% |  |
Long-term Debt | $ 40.0 |  | 40.0
First Mortgage Bonds - 4.85% Series Due 2021 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate | 4.85% |  |
Long-term Debt | $ 15.0 |  | 15.0
First Mortgage Bonds - 3.02% Series Due 2021 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate | 3.02% |  |
Long-term Debt | $ 60.0 |  | 60.0
First Mortgage Bonds - 3.40% Series Due 2022 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate | 3.40% |  |
Long-term Debt | $ 75.0 |  | 75.0
First Mortgage Bonds - 6.02% Series Due 2023 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate | 6.02% |  |
Long-term Debt | $ 75.0 |  | 75.0
First Mortgage Bonds - 3.69% Series Due 2024 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate | 3.69% |  |
Long-term Debt | $ 60.0 |  | 60.0
First Mortgage Bonds - 4.90% Series Due 2025 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate | 4.90% |  |
Long-term Debt | $ 30.0 |  | 30.0
First Mortgage Bonds - 5.10% Series Due 2025 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate | 5.10% |  |
Long-term Debt | $ 30.0 |  | 30.0
First Mortgage Bonds - 3.20% Series Due 2026 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate | 3.20% |  |
Long-term Debt | $ 75.0 |  | 75.0
First Mortgage Bonds - 5.99% Series Due 2027 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate | 5.99% |  |
Long-term Debt | $ 60.0 |  | 60.0
First Mortgage Bonds - 3.30% Series Due 2028 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate | 3.30% |  |
Long-term Debt | $ 40.0 |  | 40.0
First Mortgage Bonds - 3.74% Series Due 2029 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate | 3.74% |  |
Long-term Debt | $ 50.0 |  | 50.0
First Mortgage Bonds - 3.86% Series Due 2030 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate | 3.86% |  |
Long-term Debt | $ 60.0 |  | 60.0
First Mortgage Bonds - 5.69% Series Due 2036 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate | 5.69% |  |
Long-term Debt | $ 50.0 |  | 50.0
First Mortgage Bonds - 6.00% Series Due 2040 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate | 6.00% |  |
Long-term Debt | $ 35.0 |  | 35.0
First Mortgage Bonds - 5.82% Series Due 2040 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate | 5.82% |  |
Long-term Debt | $ 45.0 |  | 45.0
First Mortgage Bonds - 4.08% Series Due 2042 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate | 4.08% |  |
Long-term Debt | $ 85.0 |  | 85.0
First Mortgage Bonds - 4.21% Series Due 2043 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate | 4.21% |  |
Long-term Debt | $ 60.0 |  | 60.0
First Mortgage Bonds - 4.95% Series Due 2044 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate | 4.95% |  |
Long-term Debt | $ 40.0 |  | 40.0
First Mortgage Bonds - 5.05% Series Due 2044 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate | 5.05% |  |
Long-term Debt | $ 40.0 |  | 40.0
First Mortgage Bonds - 4.39% Series Due 2044 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate | 4.39% |  |
Long-term Debt | $ 50.0 |  | 50.0
ALLETE Bonds 4.07% Due April 2048 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate | 4.07% | 4.07% |
Long-term Debt | $ 60.0 |  | 0.0
Variable Demand Revenue Refunding Bonds Series 1997 A Due 2020 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term Debt | 13.5 |  | 13.5
ALLETE Term Loan Variable Rate Due 2020 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term Debt | $ 10.0 |  | 40.0
Debt Instrument, Description of Variable Rate Basis | LIBOR |  |
Debt Instrument, Basis Spread on Variable Rate | 1.025% |  |
Armenia Mountain Senior Secured Notes 3.26% Due 2024 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate | 3.26% |  |
Long-term Debt | $ 57.2 |  | 65.9
A415SeriesDue2028 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate | 4.15% |  |
Industrial Development Variable Rate Demand Refunding Revenue Bonds Series 2006, Due 2025 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term Debt | $ 27.8 |  | 27.8
ALLETE Senior Unsecured Notes 3.11% Due 2027 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate | 3.11% |  |
Long-term Debt | $ 80.0 |  | 80.0
SWL&P First Mortgage Bonds 4.15% Series Due 2028 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term Debt | $ 15.0 |  | 15.0
SWL&P First Mortgage Bonds 4.14% Series Due 2048 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate | 4.14% |  |
Long-term Debt | $ 12.0 |  | 0.0
Other Long-Term Debt, 3.11% – 5.75% Due 2019 – 2037 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term Debt | $ 67.7 |  | $ 69.1
Minimum [Member] | Other Long-Term Debt, 3.11% – 5.75% Due 2019 – 2037 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate | 3.11% |  |
Maximum [Member] | Other Long-Term Debt, 3.11% – 5.75% Due 2019 – 2037 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate | 5.75% |  |

X
- Definition Percentage points added to the reference rate to compute the variable rate on the debt instrument. + References No definition available. + Details
Name: | us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=109500613&loc=SL6031897-161870 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(16)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(22)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(16)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
Name: | us-gaap_DebtInstrumentCarryingAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The reference rate for the variable rate of the debt instrument, such as LIBOR or the US Treasury rate and the maturity of the reference rate used, such as three months or six months LIBOR. + References No definition available. + Details
Name: | us-gaap_DebtInstrumentDescriptionOfVariableRateBasis
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Contractual interest rate for funds borrowed, under the debt agreement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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_DebtInstrumentLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.17) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28555-108399 + Details
Name: | us-gaap_DeferredFinanceCostsNet
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (b)(3) -URI http://asc.fasb.org/extlink&oid=109500613&loc=SL6031897-161870 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(16)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(16)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(22)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_LongTermDebt
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt, classified as current. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19,20) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_LongTermDebtCurrent
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_LongTermDebtNoncurrent
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Details
Name: | us-gaap_DebtInstrumentAxis=ale_AlleteBonds183DueApril2018Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=ale_A817SeriesDue2019Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=ale_A528SeriesDue2020Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=ale_ALLETEBonds2.80DueSeptember2020Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=ale_A485SeriesDue2021Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=ale_ALLETEBonds3.02DueSeptember2021Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=ale_ALLETEBonds3.40DueJuly2022Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=ale_A602SeriesDue2023Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=ale_ALLETEBonds3.69DueMarch2024Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=ale_A490SeriesDue2025Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=ale_A510SeriesDue2025Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=ale_AlleteBonds320DueJuly2026Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=ale_A599SeriesDue2027Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=ale_AlleteBonds330DueOctober2028Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=ale_ALLETEBonds3.74DueSeptember2029Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=ale_ALLETEBonds3.86DueSeptember2030Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=ale_A569SeriesDue2036Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=ale_A600SeriesDue2040Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=ale_A582SeriesDue2040Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=ale_AlleteBonds408DueJuly2042Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=ale_AlleteBonds421DueOctober2043Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=ale_ALLETEBonds4.95DueMarch2044Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=ale_ALLETEBonds5.05DueJuly2044Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=ale_ALLETEBonds4.39DueSeptember2044Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=ale_ALLETEBonds4.07DueApril2048Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=ale_VariableDemandRevenueRefundingBondsSeries1997AbcDue20132020Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=ale_ALLETETermLoanVariableRateDue2020Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=ale_ArmeniaMountainSeniorSecuredNotes3.26Due2024Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=ale_A415SeriesDue2028Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=ale_IndustrialDevelopmentVariableRateDemandRefundingRevenueBondsSeries2006Due2025Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=ale_ALLETESeniorUnsecuredNotes3.11Due2027Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=ale_SWLPFirstMortgageBonds4.15SeriesDue2028Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=ale_SWLPFirstMortgageBonds4.14SeriesDue2048Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=ale_OtherLongTermDebtMember
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: |