Operations and Significant Accounting Policies - Balance Sheet and Income Statement Disclosures (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and Cash Equivalents		$ 44.3	$ 69.3	$ 69.1
Restricted Cash, Noncurrent		20.1	20.4	8.6
Restricted Cash included in Prepayments and Other		0.8	2.8	1.3
Cash, Cash Equivalents and Restricted Cash on the Consolidated Statement of Cash Flows		65.2	92.5	79.0	$ 110.1
Accounts Receivable [Abstract]
Billed		93.5	77.2
Unbilled		20.9	20.1
Less: Allowance for Doubtful Accounts		2.5	0.9
Total Accounts Receivable		111.9	96.4
Inventories – Net [Abstract]
Fuel	[1]	23.1	25.9
Materials and Supplies		51.1	46.9
Total Inventories		74.2	72.8
Impairment of Long-Lived Assets [Abstract]
Impairment of Real Estate		0.0	0.0	0.0
Other Non-Current Assets [Abstract]
Contract Asset - Noncurrent	[2]	25.5	28.0
Operating Lease, Right-of-Use Asset		22.4	28.6
Real Estate Investments, Net		18.2	21.9
Restricted Cash, Noncurrent		20.1	20.4	8.6
Other Postretirement Benefit Plans		34.2	37.5
Other		86.4	81.8
Total Other Non-Current Assets		206.8	218.2
PSAs		12.5	12.3
Public Utilities, Fuel Adjustment Clause	[3]	3.7	0.0
Other		44.6	41.2
Other		66.7	60.4
Asset Retirement Obligation		166.6	160.3
PSAs		52.1	64.6
Other		50.1	46.3
Other Non-Current Liabilities		285.3	293.0
Business Segments [Line Items]
Other – Non-utility		11.3	11.6	23.6
Operating Expenses – Other [Abstract]
Change in Fair Value of Contingent Consideration		0.0	0.0	(2.0)
Total Operating Expenses – Other		$ 0.0	$ 0.0	(2.0)
Income Taxes [Abstract]
More-Likely-Than-Not Percentage		50.00%	50.00%
Corporate and Other [Member]
Operating Expenses – Other [Abstract]
Total Operating Expenses – Other	[4]	$ 0.0	$ 0.0	$ (2.0)
MPUC [Member] | Taconite Harbor [Member] | Resource Package [Member]
Property, Plant and Equipment [Line Items]
Net Book Value		50.0
MPUC [Member] | Boswell Unit 3 [Member] | 2021 Integrated Resource Plan [Member]
Property, Plant and Equipment [Line Items]
Net Book Value		255.0
Other Current Liabilities
Operating Lease, Liability, Current		5.9	6.9
Other Noncurrent Liabilities
Operating Lease, Liability, Noncurrent		$ 16.5	$ 21.8

[1] Fuel consists primarily of coal inventory at Minnesota Power.

[2] Contract Assets include payments made to customers as an incentive to execute or extend service agreements. The contract payments are being amortized over the term of the respective agreements as a reduction to revenue.

[3] See Note 4. Regulatory Matters.

[4] See Note 1. Operations and Significant Accounting Policies.