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Short-Term and Long-Term Debt (Details) - USD ($) $ in Millions | Apr. 16, 2018 | Jun. 30, 2018 | Dec. 31, 2017
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Debt Instrument [Line Items] |  |  |
Short-Term Debt - Principal |  | $ 57.1 | $ 64.6
Short-Term Debt - Unamortized Debt Issuance Costs |  | (0.4) | (0.5)
Short-Term Debt - Total |  | 56.7 | 64.1
Long-Term Debt - Principal |  | 1,471.7 | 1,448.7
Long-Term Debt - Unamortized Debt Issuance Costs |  | (9.5) | (9.5)
Long-Term Debt - Total |  | 1,462.2 | 1,439.2
Total Debt - Principal |  | 1,528.8 | 1,513.3
Total Debt - Unamortized Debt Issuance Costs |  | (9.9) | (10.0)
Total Debt - Total |  | $ 1,518.9 | $ 1,503.3
ALLETE First Mortgage Bonds 4.07% Due April 2048 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
First Mortgage Bonds - Proceeds from Sale | $ 60.0 |  |
First Mortgage Bonds - Interest Rate | 4.07% |  |

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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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 14 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3255-108585 + Details
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