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Pension and Other Postretirement Benefit Plans - Plan Asset Allocations (Details) - shares shares in Millions | Dec. 31, 2018 | Dec. 31, 2017
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Pension [Member] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |  |
Actual Plan Asset Allocations |  | 100.00% | 100.00%
ALLETE Common Stock Included in Pension Plan Equity Securities (Shares) |  | 0.0 | 0.0
Plan Asset Target Allocations |  | 100.00% |
Pension [Member] | Equity Securities [Member] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |  |
Actual Plan Asset Allocations |  | 32.00% | 53.00%
Plan Asset Target Allocations |  | 32.00% |
Pension [Member] | Fixed Income Securities [Member] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |  |
Actual Plan Asset Allocations |  | 60.00% | 38.00%
Plan Asset Target Allocations |  | 56.00% |
Pension [Member] | Private Equity [Member] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |  |
Actual Plan Asset Allocations |  | 5.00% | 5.00%
Plan Asset Target Allocations |  | 6.00% |
Pension [Member] | Real Estate [Member] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |  |
Actual Plan Asset Allocations |  | 3.00% | 4.00%
Plan Asset Target Allocations |  | 6.00% |
Postretirement Health and Life [Member] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |  |
Actual Plan Asset Allocations | [1] | 100.00% | 100.00%
Plan Asset Target Allocations | [2] | 100.00% |
Postretirement Health and Life [Member] | Equity Securities [Member] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |  |
Actual Plan Asset Allocations | [1] | 62.00% | 64.00%
Plan Asset Target Allocations | [2] | 60.00% |
Postretirement Health and Life [Member] | Fixed Income Securities [Member] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |  |
Actual Plan Asset Allocations | [1] | 34.00% | 31.00%
Plan Asset Target Allocations | [2] | 37.00% |
Postretirement Health and Life [Member] | Private Equity [Member] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |  |
Actual Plan Asset Allocations | [1] | 4.00% | 5.00%
Plan Asset Target Allocations | [2] | 3.00% |
Postretirement Health and Life [Member] | Real Estate [Member] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |  |
Actual Plan Asset Allocations | [1] | 0.00% | 0.00%
Plan Asset Target Allocations | [2] | 0.00% |
[1] | Includes VEBAs and irrevocable grantor trusts.
[2] | Includes VEBAs and irrevocable grantor trusts.

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- Definition Number of shares of employer stock in which defined benefit plan asset is invested. Includes, but is not limited to, stock issued or managed by related party of employer. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (n) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 + Details
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- Definition Percentage of target investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one plan. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 55 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=112275985&loc=d3e4179-114921 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(i) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 + Details
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- Definition Percentage of actual investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one plan. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(i) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 + Details
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