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Commitments, Guarantees and Contingencies - Other Matters (Details) - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2018 | Dec. 31, 2017
ALLETE Clean Energy [Member] | Letters of Credit [Member] |  |
Guarantor Obligations [Line Items] |  |
Collateral | $ 21.0 |
U.S. Water Services [Member] | Letters of Credit [Member] |  |
Guarantor Obligations [Line Items] |  |
Collateral | 0.0 |
BNI Energy Reclamation Liability [Member] |  |
Guarantor Obligations [Line Items] |  |
Estimated Obligation | 47.5 |
BNI Energy Reclamation Liability [Member] | Letters of Credit [Member] |  |
Guarantor Obligations [Line Items] |  |
Collateral | 0.6 |
BNI Energy Reclamation Liability [Member] | Surety Bonds [Member] |  |
Guarantor Obligations [Line Items] |  |
Collateral | 49.9 |
ALLETE Properties Development and Maintenance Obligations [Member] |  |
Guarantor Obligations [Line Items] |  |
Estimated Obligation | 6.1 |
ALLETE Properties Development and Maintenance Obligations [Member] | Surety Bonds and Letters of Credit [Member] |  |
Guarantor Obligations [Line Items] |  |
Collateral | 8.6 |
Town Center District Capital Improvement Bonds [Member] |  |
Guarantor Obligations [Line Items] |  |
Bonds | $ 26.4 |
Bond Interest Rate | 6.00% |
Bond Term (Years) | 31 years |
Ownership Percentage of Benefited Property | 68.00% | 70.00%
Annual Assessment | $ 1.4 |
Palm Coast Park District Special Assessment Bonds [Member] |  |
Guarantor Obligations [Line Items] |  |
Bonds | $ 31.8 |
Bond Interest Rate | 5.70% |
Bond Term (Years) | 31 years |
Ownership Percentage of Benefited Property | 19.00% | 33.00%
Annual Assessment | $ 0.6 |

X
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- Definition Interest rate of capital improvement revenue and special assessment bonds used to fund real estate development projects. + References No definition available. + Details
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- Definition Ownership percentage of property benefitied by a real estate development project funded by municipal capital improvement revenue and special assessment bonds. + References No definition available. + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 460 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=115931803&loc=d3e12069-110248 + Details
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