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Pension and Other Postretirement Benefit Plans - Contributions (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended | 3 Months Ended |  | 3 Months Ended | 12 Months Ended | 3 Months Ended | 12 Months Ended
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Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2012 Pension [Member] | Dec. 31, 2011 Pension [Member] | Mar. 31, 2013 Postretirement Health and Life [Member] | Feb. 15, 2013 Postretirement Health and Life [Member] | Mar. 31, 2013 VEBA [Member] | Dec. 31, 2012 VEBA [Member] | Dec. 31, 2011 VEBA [Member] | Mar. 31, 2013 Grantor Trust [Member] | Dec. 31, 2012 Grantor Trust [Member] | Dec. 31, 2011 Grantor Trust [Member]
Pension and Other Postretirement Benefit Plans [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Defined Contribution Plan, Employer Matching Contribution, Amount | $ 7.7 | $ 7.3 |  |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Defined Benefit Plans, Employer Contributions |  |  |  | 7.3 | 33.8 | 4.8 |  | 2.8 | 1.5 | 10.9 | 2.0 | 0 | 0
ALLETE Common Stock Contributed to Pension Plan | 0 | (20.0) | 0 |  | 20.0 |  |  |  |  |  |  |  |
Defined Benefit Plans, Estimated Employer Contributions |  |  |  | $ 0 |  |  | $ 0 |  |  |  |  |  |

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- Definition Amount employer contributed during the period, to match employee contributions, to a defined contribution plan. Includes, but not limited to, contributions made through Employee Stock Ownership Plans (ESOP). + References No definition available. + Details
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- Definition The employer's best estimate, as soon as it can be reasonably determined, of contributions expected to be paid to the plan during the next fiscal year beginning after the date of the latest statement of financial position. Estimated contributions may be presented in the aggregate combining (1) contributions required by funding regulations or laws, (2) discretionary contributions, and (3) noncash contributions. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=21915506&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 132R -Paragraph 5 -Subparagraph g -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. + Details
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- Definition The amount of cash or cash equivalents contributed during the reporting period by the entity to fund its pension plans and its non-pension postretirement benefit plans. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 28 -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 230 -SubTopic 10 -Section 45 -Paragraph 25 -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3536-108585 + Details
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- Definition Value of shares issued during the period to an employee benefit plan, such as a defined contribution or defined benefit plan. + References No definition available. + Details
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