Regulatory Matters - Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets and Liabilities Currently Earning a Return		With the exception of the regulatory asset for Boswell Units 1 and 2, no other regulatory assets are currently earning a return.
Non-Current Regulatory Assets		$ 389.5	$ 384.7
Current Regulatory Liability		55.1	0.0
Non-Current Regulatory Liabilities		512.1	532.0
Total Regulatory Liabilities		567.2	532.0
Minnesota Power [Member] | Retail Customers [Member] | Electric Rates [Member] | Energy-Intensive Trade-Exposed Customer Rates [Member] | MPUC [Member]
Regulatory Assets and Liabilities [Line Items]
Provided Discounts to EITE Customers to be Offset Against Interim Rate Refunds		23.8	8.6
Provision for Interim Rate Refund [Domain]
Regulatory Assets and Liabilities [Line Items]
Current Regulatory Liability	[1],[2]	40.0	0.0
Provision for Tax Reform Refund [Domain]
Regulatory Assets and Liabilities [Line Items]
Current Regulatory Liability	[1],[3]	10.7	0.0
Transmission Formula Rates [Member]
Regulatory Assets and Liabilities [Line Items]
Current Regulatory Liability	[1]	4.4	0.0
Non-Current Regulatory Liabilities		1.6	0.0
Income Taxes [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities	[4]	396.4	411.2
Wholesale and Retail Contra AFUDC [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities	[5]	64.4	57.9
Revenue Subject to Refund [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities	[2]	0.0	23.7
Plant Removal Obligations [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		25.1	20.3
North Dakota Investment Tax Credits [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities	[6]	14.7	14.1
Cost Recovery Riders [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities	[7]	6.9	2.2
Other [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		3.0	2.6
Defined Benefit Pension and Other Postretirement Benefit Plans [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets	[8]	218.5	220.3
Income Taxes [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets	[4]	105.5	112.8
Asset Retirement Obligation [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets	[9]	32.6	29.6
Boswell 1 & 2 [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets	[10]	16.3	0.0
Manufactured Gas Plant [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets	[11]	8.0	8.1
PPACA Income Tax Deferral [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		5.0	5.0
CIP Financial Incentive [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets	[12]	0.0	3.3
Other [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		$ 3.6	$ 5.6
Regulatory Clause Revenues, under-recovered [Member]
Regulatory Assets and Liabilities [Line Items]
Rider Revenue Recovery Collection Period (Years)		2 years

[1] Current regulatory liabilities are presented within Other Current Liabilities on the Consolidated Balance Sheet.

[2] This amount is expected to be refunded to Minnesota Power’s regulated retail customers in 2019 and includes $23.8 million of discounts provided to EITE customers as of December 31, 2018, that will be offset against interim rate refunds ($8.6 million as of December 31, 2017). (See 2016 Minnesota General Rate Case and Energy-Intensive Trade‑Exposed Customer Rates.)

[3] Provision for tax reform refund is expected to be refunded to Minnesota Power customers in the first quarter of 2019 and SWL&P customers in 2019 and 2020. (See Tax Cuts and Jobs Act of 2017.)

[4] These costs represent the difference between deferred income taxes recognized for financial reporting purposes and amounts previously billed to our customers. The balances will primarily decrease over the remaining life of the related temporary differences and flow through current income taxes.

[5] Wholesale and retail contra AFUDC represents amortization to offset AFUDC Equity and Debt recorded during the construction period of our cost recovery rider projects prior to placing the projects in service. The regulatory liability will decrease over the remaining depreciable life of the related asset.

[6] North Dakota investment tax credits expected to be realized from Bison that will be credited to Minnesota Power’s regulated retail customers through future renewable cost recovery rider filings as the tax credits are utilized.

[7] The cost recovery rider regulatory liabilities are cash collections from our customers in excess of revenue recognized, primarily due to capital expenditures related to Bison, investment in CapX2020 projects, the Boswell Unit 4 environmental upgrade and the GNTL. The cost recovery rider regulatory liabilities as of December 31, 2018, will be returned within the next two years.

[8] Defined benefit pension and other postretirement items included in our Regulated Operations, which are otherwise required to be recognized in accumulated other comprehensive income as actuarial gains and losses as well as prior service costs and credits, are recognized as regulatory assets or regulatory liabilities on the Consolidated Balance Sheet. The asset or liability will decrease as the deferred items are amortized and recognized as components of net periodic benefit cost. (See Note 15. Pension and Other Postretirement Benefit Plans.)

[9] Asset retirement obligations will accrete and be amortized over the lives of the related property with asset retirement obligations.

[10] In December 2018, Minnesota Power retired Boswell Units 1 and 2 and reclassified the remaining net book value from property, plant and equipment to a regulatory asset on the Consolidated Balance Sheet. The remaining net book value is currently included in Minnesota Power’s rate base and Minnesota Power is earning a return on the outstanding balance.

[11] The manufactured gas plant regulatory asset represents costs of remediation for a former manufactured gas plant site located in Superior, Wisconsin, and formerly operated by SWL&P. We expect recovery of these remediation costs to be allowed by the PSCW in rates over time.

[12] The conservation improvement program regulatory asset represents CIP expenditures, any financial incentive earned for cost-effective program achievements and a carrying charge deferred for future cost recovery over the next year following MPUC approval.