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Short-Term and Long-Term Debt (Details) (USD $) In Millions, unless otherwise specified | 0 Months Ended | 3 Months Ended |  |  |  |  |  | 0 Months Ended |  | 0 Months Ended
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Dec. 10, 2013 | Apr. 02, 2013 | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2013 Lines of Credit [Member] | Dec. 31, 2012 Lines of Credit [Member] | Nov. 04, 2013 ALLETE LOC Nov2018 [Member] [Member] | Nov. 04, 2013 ALLETE LOC June 2015 [Member] | Nov. 04, 2013 ALLETE LOC January 2014 [Member] | Dec. 31, 2012 ALLETE Bonds 4.86% Due 2013 [Member] | Apr. 02, 2013 ALLETE Bonds 1.83% Due April2018 [Member] | Apr. 02, 2013 ALLETE Bonds 3.30% Due October2028 [Member] | Apr. 02, 2013 ALLETE Bonds 4.21% Due October2043 [Member] | Aug. 26, 2013 ALLETE Amended Term Loan Variable Rate Due August 2015 [Member] | Aug. 26, 2013 ALLETE Amended Term Loan Variable Rate Due 2015 Effective August 25, 2013 [Member] | Aug. 26, 2013 ALLETE Amended Term Loan Variable Rate Due 2015 Effective August 26, 2014 [Member] | Dec. 10, 2013 ALLETE Bonds 3.69% Due March2024 [Member] | Dec. 10, 2013 ALLETE Bonds 4.95% Due March2044 [Member] | Dec. 10, 2013 ALLETE Bonds 3.40% Due July2022 [Member] | Dec. 10, 2013 ALLETE Bonds 5.05% Due July2044 [Member]
Financial Covenants [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Required Indebtedness to Total Capital Ratio |  |  | 0.65 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Actual Indebtedness to Total Capital Ratio |  |  | 0.45 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maturities of Long-term Debt [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt |  |  | $ 1,083.0 | $ 933.6 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt Due Within One Year |  |  | 27.2 | 84.5 |  |  |  |  |  | 60.0 |  |  |  |  |  |  |  |  |  |
Long-Term Debt, 2014 Maturities |  |  | 143.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt, 2015 Maturities |  |  | 22.3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt, 2016 Maturities |  |  | 51.8 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt, 2017 Maturities |  |  | 1.7 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt, Thereafter Maturities |  |  | 864.2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-Term Debt |  |  | 27.2 | 84.5 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility, Maximum Borrowing Capacity |  |  |  |  | 406.4 | 406.4 | 400.0 | 250.0 | 150.0 |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility Additional Borrowing Capacity |  |  |  |  |  |  | 150.0 |  |  |  |  |  |  |  |  |  |  |  |  |
Letter of Credit Maximum Issuances |  |  |  |  |  |  | 60.0 |  |  |  |  |  |  |  |  |  |  |  |  |
Lines of Credit, Current |  |  | 5.4 | 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from Issuance of First Mortgage Bond |  | 150.0 |  |  |  |  |  |  |  |  | 50.0 | 40.0 | 60.0 |  |  |  |  |  |  |
First Mortgage Bonds, Interest Rate |  |  |  |  |  |  |  |  |  |  | 1.83% | 3.30% | 4.21% |  |  |  |  |  |  |
Expected Proceeds from Future Issuance of First Mortgage Bond | 215.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 60.0 | 40.0 | 75.0 | 40.0
First Mortgage Bonds, Interest Rate, Future Issuance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3.69% | 4.95% | 3.40% | 5.05%
Debt Instrument, Face Amount |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 75.0 |  |  |  |  |  |
Debt Instrument, Description of Variable Rate Basis |  |  |  |  |  |  |  |  |  |  |  |  |  | one-month LIBOR |  |  |  |  |  |
Debt Instrument, Basis Spread on Variable Rate |  |  |  |  |  |  |  |  |  |  |  |  |  | 0.875% |  |  |  |  |  |
Debt Instrument, Interest Rate, Effective Percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1.70% | 1.625% |  |  |  |

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- Definition Indebtedness to total capital ratio required to be maintained by most restrictive financial covenant. + References No definition available. + Details
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- Definition The expected cash inflow from amounts received from a future long-term debt instrument issuance secured by a first mortgage deed of trust, containing a pledge of real property. The lender has the highest claim on the property in case of default. + References No definition available. + Details
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- Definition This represents the allowed amount of letters of credit issuances under another credit agreement. + References No definition available. + Details
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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://www.xbrl.org/2003/role/presentationRef -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 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19 -Article 5 + Details
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- Definition Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 22 -Article 5 Reference 3: http://www.xbrl.org/2003/role/presentationRef -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
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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://www.xbrl.org/2003/role/presentationRef -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 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 22 -Article 5 + 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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.20) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 20 -Article 5 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Line-of-Credit Arrangement -URI http://asc.fasb.org/extlink&oid=6517033 + Details
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- Definition Amount of long-term debt, after unamortized discount or premium, scheduled 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://www.xbrl.org/2003/role/presentationRef -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 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 20 -Article 5 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19 -Article 5 + Details
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- Definition Amount of long-term debt, sinking fund requirements, and other securities redeemable at fixed or determinable prices and dates maturing after the fifth fiscal year following the latest fiscal year. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601 + Details
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- Definition Amount of long-term debt, sinking fund requirements, and other securities redeemable at fixed or determinable prices and dates maturing in the fourth fiscal year following the latest fiscal year. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601 + Details
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- Definition Amount of long-term debt, sinking fund requirements, and other securities redeemable at fixed or determinable prices and dates maturing in the third fiscal year following the latest fiscal year. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601 + Details
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- Definition Amount of long-term debt, sinking fund requirements, and other securities redeemable at fixed or determinable prices and dates maturing in the second fiscal year following the latest fiscal year. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601 + Details
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- Definition Carrying amount of long-term debt, net of unamortized discount or premium, excluding amounts to be repaid within one year or the normal operating cycle, if longer (current maturities). 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/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 22 -Article 5 Reference 2: http://www.xbrl.org/2003/role/presentationRef -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
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- Definition The cash inflow from amounts received from a long-term debt instrument secured by a first mortgage deed of trust, containing a pledge of real property. The lender has the highest claim on the property in case of default. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Financing Activities -URI http://asc.fasb.org/extlink&oid=6513228 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 14 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3255-108585 + Details
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