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Commitments, Guarantees and Contingencies - Power Purchase Agreements (Details) MWh in Millions, $ in Millions		12 Months Ended
		Dec. 31, 2017 USD ($) MWh MW	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Jul. 28, 2017 MW
Basin [Member] | Basin PSA (expires April 2020) [Member]
Power Purchase Agreements [Line Items]
Output Being Sold (MW)		100
Basin [Member] | Basin PSA (expires 2018) [Member]
Power Purchase Agreements [Line Items]
Output Being Sold (MW)		100
Basin [Member] | Basin PSA (expires 2019) [Member]
Power Purchase Agreements [Line Items]
Output Being Sold (MW)		50
Silver Bay Power [Member] | Silver Bay Power PSA through 2031 (Years 2016-2019) [Member] | Minimum [Member]
Power Purchase Agreements [Line Items]
Output Being Sold (MW)		50
Silver Bay Power [Member] | Silver Bay Power Self-Generation [Member]
Power Purchase Agreements [Line Items]
Generating Unit Capacity (MW)		90
Great River Energy Capacity and Energy PPA (expires May 2020) [Member] | Great River Energy [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MW)		50
Great River Energy Capacity PPA Beginning June 2016 (expires May 2020) [Member] | Great River Energy [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MW)		50
Great River Energy Capacity PPA Beginning June 2017 (expires May 2020) [Member] | Great River Energy [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MW)		50
Manitoba Hydro PPA Beginning June 2015 (expires May 2020) [Member] | Manitoba Hydro [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MW)		50
Manitoba Hydro PPA Beginning June 2017 (expires May 2020) [Member] | Manitoba Hydro [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MW)		50
Manitoba Hydro PPA (expires May 2035) [Member] | Manitoba Hydro [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MW)		250
Manitoba Hydro PPA (expires 2040) [Member] | Manitoba Hydro [Member]
Power Purchase Agreements [Line Items]
Contract Term (Years)		20 years
Output Being Purchased (MW)		133
Minnkota Power PPA (expires May 2020) [Member] | Minnkota Power [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MW)		50
Shell Energy PPA (expires December 2019) [Member] | Shell Energy [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MW)		50
Manitoba Hydro PPA (expires April 2022) [Member] | Manitoba Hydro [Member] | Minimum [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MWh) | MWh		1
Oliver Wind I PPA (expires December 2031) [Member] | Oliver Wind [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MW)		50
Oliver Wind II PPA (expires December 2032) [Member] | Oliver Wind [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MW)		48
TransAlta Off-Peak Hours PPA (expires December 2019) [Member] | TransAlta [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MW)		50
TransAlta On-Peak Hours PPA (expire December 2019) [Member] | TransAlta [Member]
Power Purchase Agreements [Line Items]
Output Being Purchased (MW)		100
Tenaska PPA [Member] | Tenaska [Member] | Wind Turbine Generators [Member] | Resource Package [Member] | MPUC [Member]
Power Purchase Agreements [Line Items]
Generating Capacity Counterparty Owned (MW)	[1]	250
Natural Gas PPA [Member] | Jointly Owned by ALLETE and Dairyland Power Cooperative [Member] | Combined-Cycle Natural Gas-Fired Generating Facility [Member] | Natural Gas-Fired [Member] | Resource Package [Member] | MPUC [Member] | Jointly-Owned Electricity Generation Plant [Member] | Minimum [Member]
Power Purchase Agreements [Line Items]
Generating Capacity Jointly Owned (MW)		525			525
Natural Gas PPA [Member] | Jointly Owned by ALLETE and Dairyland Power Cooperative [Member] | Combined-Cycle Natural Gas-Fired Generating Facility [Member] | Natural Gas-Fired [Member] | Resource Package [Member] | MPUC [Member] | Jointly-Owned Electricity Generation Plant [Member] | Maximum [Member]
Power Purchase Agreements [Line Items]
Generating Capacity Jointly Owned (MW)		550			550
Square Butte PPA (expires 2026) [Member] | Square Butte Coal-fired Unit [Member]
Power Purchase Agreements [Line Items]
Minnesota Power Output Entitlement (Percent)		50.00%
Output Being Purchased (MW)		227.5
Square Butte PPA (expires 2026) [Member] | Minnkota Power [Member] | Square Butte Coal-fired Unit [Member] | Minnkota Power PSA (expires December 2026) [Member]
Power Purchase Agreements [Line Items]
Minnesota Power Output Entitlement (Percent)		28.00%	28.00%	28.00%
Square Butte PPA (expires 2026) [Member] | Square Butte [Member]
Power Purchase Agreements [Line Items]
PPA Counterparty Total Debt Outstanding | $		$ 330.0
PPA Counterparty Annual Debt Service through 2022 | $		49.0
Cost of Purchased Power | $		75.7	$ 73.3	$ 77.8
Pro Rata Share of PPA Counterparty Interest Expense | $		$ 9.4	$ 9.6	$ 10.1
Square Butte PPA (expires 2026) [Member] | Square Butte [Member] | Square Butte Coal-fired Unit [Member]
Power Purchase Agreements [Line Items]
Generating Unit Capacity (MW)		455
[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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