Commitments, Guarantees and Contingencies - Power Purchase Agreements (Details) $ in Millions	9 Months Ended
	Sep. 30, 2018 USD ($) MW	Sep. 30, 2017 USD ($)
Square Butte [Member] | Square Butte PPA [Member]
Power Purchase Agreements [Line Items]
PPA Counterparty Total Debt Outstanding | $	$ 308.4
Square Butte [Member] | Square Butte PPA [Member] | Minnesota Power [Member]
Power Purchase Agreements [Line Items]
Cost of Power Purchased | $	57.9	$ 60.6
Pro Rata Share of PPA Counterparty Interest Expense | $	$ 6.9	$ 7.1
Square Butte [Member] | Square Butte PPA [Member] | Square Butte Coal-fired Unit [Member]
Power Purchase Agreements [Line Items]
Generating Capacity Counterparty Owned (MW)	455
Square Butte [Member] | Square Butte PPA [Member] | Square Butte Coal-fired Unit [Member] | Minnesota Power [Member]
Power Purchase Agreements [Line Items]
Expected Output Entitlement	50.00%
Minnkota Power [Member] | Square Butte PPA [Member] | Square Butte Coal-fired Unit [Member] | Minnesota Power [Member] | Minnkota Power PSA [Member]
Power Purchase Agreements [Line Items]
Expected Output Entitlement	28.00%	28.00%
Oconto Electric Cooperative [Member] | Minnesota Power [Member] | Oconto Electric Cooperative PSA [Member]
Power Purchase Agreements [Line Items]
Energy and Capacity Expected to be Provided (MW)	25
Calpine Corporation [Member] | Minnesota Power [Member] | Calpine Corporation Power Purchase Agreement [Member]
Power Purchase Agreements [Line Items]
Capacity Purchased (MW)	25
Basin Electric Power Cooperative [Member] | Minnesota Power [Member] | Basin Electric Cooperative PSA [Member]
Power Purchase Agreements [Line Items]
Capacity Sale (MW)	100
Cypress Creek Renewables [Member] | Cypress Creek Renewables PPA [Member] | Blanchard Solar Energy Facility [Member] | Minnesota Power [Member]
Power Purchase Agreements [Line Items]
Electric Energy and Capacity Purchased (MW)	10
Contract Length (Years)	25 years