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Short-Term and Long-Term Debt - Short-Term Debt (Details) - USD ($) $ in Millions | Jan. 10, 2019 | Dec. 31, 2018 | Dec. 31, 2017
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Debt Disclosure [Abstract] |  |  |
Short-Term Debt - Outstanding |  | $ 57.5 | $ 64.1
Short-Term Debt - Unamortized Debt Issuance Costs |  | 0.4 |
Bank Lines of Credit [Member] |  |  |
Lines of Credit [Line Items] |  |  |
Maximum Borrowing Capacity |  | 407.0 | 407.0
Standby Letters of Credit Outstanding |  | 18.4 | 11.9
Draws Outstanding |  | 0.0 | $ 0.0
ALLETE Amended and Restated Credit Agreement [Member] |  |  |
Lines of Credit [Line Items] |  |  |
Maximum Borrowing Capacity |  | $ 400.0 |
Subsequent Event [Member] | Bank Lines of Credit [Member] |  |  |
Lines of Credit [Line Items] |  |  |
Maximum Borrowing Capacity | $ 400.0 |  |
Line of Credit Facility Additional Borrowing Capacity | 150.0 |  |
Letters of Credit Maximum Issuances | $ 60.0 |  |

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- Definition Letters of Credit Maximum Issuances + References No definition available. + Details
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Balance Type: | debit
Period Type: | instant

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- Definition Amount of line of credit borrowing capacity pledged against letters of credit at the end of the period limiting the line of credit borrowing capacity but not reflected as debt on the balance sheet. + References No definition available. + Details
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Namespace Prefix: | ale_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Line of Credit Facility Additional Borrowing Capacity + References No definition available. + Details
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Namespace Prefix: | ale_
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Balance Type: | debit
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- Definition Amount of short-term debt and current maturity of long-term debt and capital lease obligations due within one year or the normal operating cycle, if longer. + 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
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- References No definition available. + Details
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- Definition Amount, after accumulated amortization, of debt issuance costs classified as current. 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 210.5-02.8) -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
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- 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
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- Definition Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility. + 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(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition The carrying value as of the balance sheet date of the current portion of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement. + References Reference 1: 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(13)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 2: 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)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Details
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- Details
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