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Goodwill and Intangible Assets - Intangible Assets (Details) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2019 | Mar. 31, 2018
Definite-Lived Intangible Assets [Roll Forward] |  |  |
Beginning Balance |  | $ 58.2 |
Amortization |  | (1.4) | $ (1.4)
Other Changes | [1] | 55.7 |
Ending Balance |  | 1.1 |
Intangible Assets [Abstract] |  |  |
Total Intangible Assets - Beginning Balance |  | 74.8 |
Total Intangible Assets - Amortization |  | (1.4) | $ (1.4)
Total Intangible Assets - Other Changes | [1] | 72.3 |
Total Intangible Assets - Ending Balance |  | 1.1 |
Trademarks and Trade Names [Member] |  |  |
Indefinite-Lived Intangible Assets [Roll Forward] |  |  |
Beginning Balance |  | 16.6 |
Other Changes | [1] | 16.6 |
Ending Balance |  | 0.0 |
Customer Relationships [Member] |  |  |
Definite-Lived Intangible Assets [Roll Forward] |  |  |
Beginning Balance |  | 50.7 |
Amortization |  | (1.1) |
Other Changes | [1] | 49.6 |
Ending Balance |  | 0.0 |
Intangible Assets [Abstract] |  |  |
Total Intangible Assets - Amortization |  | (1.1) |
Developed Technology and Other [Member] |  |  |
Definite-Lived Intangible Assets [Roll Forward] |  |  |
Beginning Balance | [2] | 7.5 |
Amortization | [2] | (0.3) |
Other Changes | [1],[2] | 6.1 |
Ending Balance | [2] | 1.1 |
Intangible Assets [Abstract] |  |  |
Total Intangible Assets - Amortization | [2] | $ (0.3) |
[1] | On March 26, 2019, ALLETE completed the sale of U.S. Water Services which resulted in the removal of the related intangible assets from the Consolidated Balance Sheet.
[2] | Developed Technology and Other includes patents, non-compete agreements, land easements and trade names with finite lives.

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- Definition Other changes to finite-lived intangible assets not disclosed separately. + References No definition available. + Details
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- Definition Other changes to indefinite-lived intangible assets not disclosed separately. + References No definition available. + Details
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- Definition The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16323-109275 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6388964&loc=d3e16225-109274 + Details
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- Definition Amount after amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16323-109275 + Details
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- Definition Amount of assets, excluding financial assets and goodwill, lacking physical substance and having a projected indefinite period of benefit. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16323-109275 + Details
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- Definition Sum of the carrying amounts of all intangible assets, excluding goodwill, as of the balance sheet date, net of accumulated amortization and impairment charges. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph ((a)(1),(b)) -URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16323-109275 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6388964&loc=d3e16212-109274 + Details
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