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Fair Value - Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items] |  |  |  |
Equity Investments |  | $ 161.1 | $ 118.7 |
Goodwill, Impairment Charge | [1] | 0.0 | 0.0 | $ 3.3
Intangible Assets, Carrying Amount |  | 74.8 | 77.6 |
Intangible Assets, Indicators of Impairment |  | 0.0 | 0.0 | 0.0
U.S. Water Services [Member] |  |  |  |
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items] |  |  |  |
Goodwill, Carrying Amount |  | 148.5 | 148.3 | 131.2
Goodwill, Impairment Charge |  | 0.0 | 0.0 | 0.0
ALLETE Clean Energy [Member] |  |  |  |
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items] |  |  |  |
Goodwill, Impairment Charge |  |  |  | 3.3
ATC [Member] |  |  |  |
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items] |  |  |  |
Equity Investments |  | 128.1 | 118.7 | 135.6
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Member] |  |  |  |
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items] |  |  |  |
Intangible Assets, Carrying Amount |  | 74.8 | 77.6 |
Intangible Assets, Indicators of Impairment |  | 0.0 |  |
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Member] | U.S. Water Services [Member] |  |  |  |
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items] |  |  |  |
Goodwill, Carrying Amount |  | 148.5 | 148.3 |
Goodwill, Impairment Charge |  | 0.0 | 0.0 | $ 0.0
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Member] | Corporate and Other [Member] |  |  |  |
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items] |  |  |  |
Property, Plant and Equipment, Indicators of Impairment |  | 0.0 |  |
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Member] | ATC [Member] | Regulated Operations [Member] |  |  |  |
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items] |  |  |  |
Equity Method Investment, Indicators of Impairment |  | $ 0.0 | $ 0.0 |
[1] | See Goodwill and Intangible Assets.

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- Definition This item represents an other than temporary decline in value that has been recognized against an investment accounted for under the equity method of accounting. The excess of the carrying amount over the fair value of the investment represents the amount of the write down which is or was reflected in earnings. The written down value is a new cost basis with the adjusted value of the investment becoming its new carrying value subject to the equity accounting method. Evidence of a loss in value might include, but would not necessarily be limited to, absence of an ability to recover the carrying amount of the investment or inability of the investee to sustain an earnings capacity which would justify the carrying amount of the investment. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 35 -Paragraph 32 -URI http://asc.fasb.org/extlink&oid=114001220&loc=d3e32787-111569 + Details
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- Definition This item represents the carrying amount on the entity's balance sheet of its investment in common stock of an equity method investee. This is not an indicator of the fair value of the investment, rather it is the initial cost adjusted for the entity's share of earnings and losses of the investee, adjusted for any distributions (dividends) and other than temporary impairment (OTTI) losses recognized. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=109237563&loc=d3e33749-111570 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 280 -SubTopic 10 -Section 50 -Paragraph 25 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8813-108599 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.12) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=77989000&loc=SL49117168-202975 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=99380562&loc=d3e13770-109266 + Details
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