Commitments, Guarantees and Contingencies - Power Purchase Agreements (Details) $ in Millions	3 Months Ended
Mar. 31, 2020 USD ($) MW	Mar. 31, 2019 USD ($)
Square Butte [Member] \| Square Butte PPA [Member]
Power Purchase Agreements [Line Items]
PPA Counterparty Total Debt Outstanding	$ 276.2
Square Butte [Member] \| Square Butte PPA [Member] \| Minnesota Power [Member]
Power Purchase Agreements [Line Items]
Cost of Power Purchased	20.1	$ 20.5
Pro Rata Share of PPA Counterparty Interest Expense	$ 1.9	$ 2.1
Square Butte [Member] \| Square Butte PPA [Member] \| Minnesota Power [Member] \| Square Butte Coal-fired Unit [Member]
Power Purchase Agreements [Line Items]
Expected Output Entitlement	50.00%
Minnkota Power [Member] \| Square Butte PPA [Member] \| Minnesota Power [Member] \| Square Butte Coal-fired Unit [Member] \| Minnkota Power PSA [Member]
Power Purchase Agreements [Line Items]
Expected Output Entitlement	28.00%	28.00%
Minnesota Power Short-term PSAs in 2020 and 2021 [Member]
Power Purchase Agreements [Line Items]
MW Sold Under Short-term PSAs \| MW	300

MW Sold Under Short-term PSAs

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- Definition

Under long-term contracts with public utility districts, cooperatives or other organizations, a utility company receives a portion of the output of a production plant constructed and financed by the district or cooperative. The utility has only a nominal or no investment at all in the plant but pays a proportionate part of the plant's costs, including debt service.

+ References

+ Details

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- Definition

The allocable portion of interest included in charges under a long-term contract to purchase electric power.

+ References

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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.

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- Definition

Share of plant output being purchased under a long-term contract to purchase electricity from a production plant constructed and financed by a public utility district, cooperative, or other organization.

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- Definition

Amount of operating expense for purchased power procured under arrangements extending beyond one year or the normal operating cycle, if longer, by regulated operation.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-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

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.2(b))
-URI http://asc.fasb.org/extlink&oid=116634182&loc=SL114868664-224227

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