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Short-Term and Long-Term Debt (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended |  |  |  |  |  | 12 Months Ended |  | 12 Months Ended |  |  |
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Dec. 31, 2011 | Aug. 25, 2011 | Dec. 31, 2010 | Feb. 01, 2012 Credit Agreement Feb 2012 [Member] | Dec. 31, 2011 Credit Agreement July 2011 [Member] | Jun. 30, 2011 Credit Agreement July 2011 [Member] | Dec. 31, 2011 Term Loan Agreement [Member] | Aug. 25, 2011 Term Loan Agreement [Member] | Dec. 31, 2011 ALLETE Properties LOC Nov 2011 [Member] | Nov. 14, 2011 ALLETE Properties LOC Nov 2011 [Member] | Sep. 30, 2011 ALLETE Properties LOC Nov 2011 [Member] | Oct. 07, 2011 ALLETE Properties LOC Oct 2011 [Member]
Financial Covenants [Abstract] |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Required Indebtedness to Total Capital Ratio | 0.65 to 1.00 |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Actual Indebtedness to Total Capital Ratio | 0.44 to 1.00 |  |  |  |  |  |  |  |  |  |  |
Maturities of Long-term Debt [Abstract] |  |  |  |  |  |  |  |  |  |  |  |
Long-term Debt, Current Maturities | $ 5.4 |  | $ 13.4 |  |  |  |  |  |  |  |  |
Long-term Debt, Maturities, Repayments of Principal in Year Two | 83.8 |  |  |  |  |  |  |  |  |  |  |
Long-term Debt, Maturities, Repayments of Principal in Year Three | 94.1 |  |  |  |  |  |  |  |  |  |  |
Long-term Debt, Maturities, Repayments of Principal in Year Four | 16.7 |  |  |  |  |  |  |  |  |  |  |
Long-term Debt, Maturities, Repayments of Principal in Year Five | 21.0 |  |  |  |  |  |  |  |  |  |  |
Long-term Debt, Maturities, Repayments of Principal after Year Five | 642.3 |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility, Maximum Borrowing Capacity |  |  |  | 150.0 | 250.0 | 150.0 |  |  |  | 8.3 | 10.0 | 3.0
Line of Credit Borrowing Capacity Reduction |  |  |  |  |  |  |  |  | 1.7 |  |  |
Letter of Credit Maximum Issuances |  |  |  | 10.0 | 40.0 |  |  |  |  |  |  |
Debt Instrument, Face Amount |  | 75.0 |  |  |  |  |  | 75.0 |  |  |  |
Debt Instrument, Description of Variable Rate Basis | one-month LIBOR |  |  |  |  |  | LIBOR |  |  |  |  |
Debt Instrument, Basis Spread on Variable Rate | 1.00% |  |  |  |  |  | 1.00% |  |  |  |  |
Debt Instrument, Interest Rate, Effective Percentage |  |  |  |  |  |  | 1.825% |  |  |  |  |
Debt Instrument, Carrying Amount at Period End |  |  |  |  |  |  | 75.0 |  |  |  |  |
Short-term Debt [Abstract] |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt | 6.5 |  | 14.4 |  |  |  |  |  |  |  |  |
Aggregate Lines of Credit, Total Borrowing Capacity | 256.4 |  | 154.0 |  |  |  |  |  |  |  |  |
Aggregate Lines of Credit, Used Borrowing Capacity | 1.1 |  | 1.0 |  |  |  |  |  |  |  |  |
Aggregate Lines of Credit, Available Borrowing Capacity | $ 255.3 |  | $ 153.0 |  |  |  |  |  |  |  |  |

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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 Statement of Financial Accounting Standard (FAS) -Number 129 -Paragraph 2, 4 -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 SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 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 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Total of the portions of the carrying amounts as of the balance sheet date of long-term debt, which may include notes payable, bonds payable, debentures, mortgage loans, and commercial paper, which are scheduled to be repaid within one year or the normal operating cycle, if longer, and after deducting unamortized discount or premiums, if any. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 20 -Article 5 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19 -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 210.5-02.19,20) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Amount of long-term debt maturities after year five following the date of the latest balance sheet presented in the financial statements, which may include maturities of long-term debt, sinking fund requirements, and other securities redeemable at fixed of determinable prices and dates. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 47 -Paragraph 10 -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 -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 maturing in year four following the date of the latest balance sheet presented in the financial statements, which may include maturities of long-term debt, sinking fund requirements, and other securities redeemable at fixed of determinable prices and dates. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 47 -Paragraph 10 -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 -Topic 470 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601 + Details
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