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Regulatory Matters - Integrated Resource Plan (Details) - MPUC [Member] - MW		Jul. 28, 2017	Dec. 31, 2017	Dec. 31, 2015
Minnesota Power [Member] | Resource Package [Member] | Natural Gas PPA [Member]
Regulatory Matters [Line Items]
Output Being Purchased (MW)		250
Natural Gas-Fired [Member] | Minnesota Power [Member] | Resource Package [Member] | Natural Gas PPA [Member]
Regulatory Matters [Line Items]
Output Purchased (Percent)		50.00%
Natural Gas-Fired [Member] | Minimum [Member] | Resource Package [Member] | Jointly Owned by ALLETE and Dairyland Power Cooperative [Member] | Natural Gas PPA [Member] | Combined-Cycle Natural Gas-Fired Generating Facility [Member] | Jointly-Owned Electricity Generation Plant [Member]
Regulatory Matters [Line Items]
Generating Capacity Jointly Owned (MW)		525	525
Natural Gas-Fired [Member] | Minimum [Member] | Minnesota Power [Member] | Integrated Resource Plan [Member]
Regulatory Matters [Line Items]
Generating Capacity Owned (MW)				200
Natural Gas-Fired [Member] | Maximum [Member] | Resource Package [Member] | Jointly Owned by ALLETE and Dairyland Power Cooperative [Member] | Natural Gas PPA [Member] | Combined-Cycle Natural Gas-Fired Generating Facility [Member] | Jointly-Owned Electricity Generation Plant [Member]
Regulatory Matters [Line Items]
Generating Capacity Jointly Owned (MW)		550	550
Natural Gas-Fired [Member] | Maximum [Member] | Minnesota Power [Member] | Integrated Resource Plan [Member]
Regulatory Matters [Line Items]
Generating Capacity Owned (MW)				300
Wind Turbine Generators [Member] | Resource Package [Member] | Tenaska [Member] | Tenaska PPA [Member]
Regulatory Matters [Line Items]
Generating Capacity Counterparty Owned (MW)	[1]		250
Wind Turbine Generators [Member] | Minnesota Power [Member] | Resource Package [Member] | Tenaska [Member] | Tenaska PPA [Member]
Regulatory Matters [Line Items]
Generating Capacity Counterparty Owned (MW)		250
Solar Energy Generation [Member] | Minnesota Power [Member] | Resource Package [Member] | Solar Energy PPA [Member]
Regulatory Matters [Line Items]
Generating Capacity Counterparty Owned (MW)		10
[1] The PPA provides for the purchase of all output from a 250 MW wind energy facility to be constructed in southwest Minnesota and is subject to approval of the construction of a 525 MW to 550 MW combined-cycle natural gas-fired facility. (See Note 4. Regulatory Matters.)

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 10.D)
-URI http://asc.fasb.org/extlink&oid=6499423&loc=d3e659339-123030

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