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Short-Term and Long-Term Debt (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended |  |  |  |  |  |  |  | 12 Months Ended
--- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2012 Lines of Credit [Member] | Dec. 31, 2011 Lines of Credit [Member] | Dec. 31, 2012 ALLETE LOC January 2014 [Member] | Jun. 30, 2012 ALLETE LOC January 2014 [Member] | Feb. 01, 2012 ALLETE LOC January 2014 [Member] | Dec. 31, 2012 ALLETE LOC June 2015 [Member] | Dec. 31, 2012 ALLETE Bonds 4.86% Due 2013 [Member] | Dec. 31, 2012 ALLETE Bonds 3.20% Due July 2026 [Member] | Dec. 31, 2012 ALLETE Bonds 4.08% Due July2042 [Member] | Dec. 31, 2012 Industrial Development Revenue Bonds 6.5% Due 2025 [Member]
Financial Covenants [Abstract] |  |  |  |  |  |  |  |  |  |  |  |
Required Indebtedness to Total Capital Ratio | 0.65 |  |  |  |  |  |  |  |  |  |  |
Actual Indebtedness to Total Capital Ratio | 0.46 |  |  |  |  |  |  |  |  |  |  |
Maturities of Long-term Debt [Abstract] |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt | $ 933.6 | $ 857.9 |  |  |  |  |  |  |  |  |  |
Long-Term Debt Due Within One Year | 84.5 | 5.4 |  |  |  |  |  |  | 60.0 |  |  |
Long-Term Debt, 2014 Maturities | 94.8 |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt, 2015 Maturities | 17.4 |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt, 2016 Maturities | 21.7 |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt, 2017 Maturities | 51.2 |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt, Thereafter Maturities | 748.5 |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Short-Term Debt | 84.5 | 6.5 |  |  |  |  |  |  |  |  |  |
Line of Credit Facility, Maximum Borrowing Capacity |  |  | 406.4 | 256.4 | 150.0 |  | 150.0 | 250.0 |  |  |  |
Letter of Credit Maximum Issuances | 50.0 |  |  |  |  |  | 10.0 |  |  |  |  |
Lines of Credit, Current | 0 | 1.1 |  |  |  |  |  |  |  |  |  |
Proceeds from Issuance of First Mortgage Bond | 160.0 |  |  |  |  |  |  |  |  | 75.0 | 85.0 |
First Mortgage Bonds, Interest Rate |  |  |  |  |  |  |  |  |  | 3.20% | 4.08% | 6.50%
Redemption of Bond |  |  |  |  |  |  |  |  |  |  |  | 6.0
Line of Credit Facility, Amount Outstanding |  |  |  |  |  | $ 14.0 |  |  |  |  |  |

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- Definition Amount borrowed under the credit facility as of the balance sheet date. + References Reference 1: 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 Carrying amount of long-term debt, net of 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 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 Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 19 -Subparagraph b -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: 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 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 18 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 4: 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=6943989&loc=d3e3255-108585 + Details
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