Regulatory Matters - Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018
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 net plant and equipment, no other regulatory assets are currently earning a return.
Non-Current Regulatory Assets		$ 420.5	$ 389.5
Current Regulatory Liability		1.9	55.1
Non-Current Regulatory Liabilities		560.3	512.1
Total Regulatory Liabilities		562.2	567.2
Provision for Interim Rate Refund [Domain]
Regulatory Assets and Liabilities [Line Items]
Current Regulatory Liability	[1],[2]	0.0	40.0
Provision for Tax Reform Refund [Domain]
Regulatory Assets and Liabilities [Line Items]
Current Regulatory Liability	[1],[3]	0.2	10.7
Transmission Formula Rates [Member]
Regulatory Assets and Liabilities [Line Items]
Current Regulatory Liability		1.7	4.4
Non-Current Regulatory Liabilities		0.0	1.6
Income Taxes [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities	[4]	407.2	396.4
Wholesale and Retail Contra AFUDC [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities	[5]	79.3	64.4
Plant Removal Obligations [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities	[6]	35.5	25.1
Pension and Other Postretirement Plans Costs [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities	[7]	17.0	0.0
North Dakota Investment Tax Credits [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities	[8]	12.3	14.7
Conservation Improvement Program [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities	[9]	5.4	1.5
Cost Recovery Riders [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities	[10]	0.0	6.9
Other [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		3.6	1.5
Pension and Other Postretirement Plans Costs [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets	[7]	212.9	218.5
Income Taxes [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets	[4]	123.4	105.5
Asset Retirement Obligation [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets	[11]	32.0	32.6
Boswell 1 & 2 [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets	[12]	10.7	16.3
Manufactured Gas Plant [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets	[13]	8.2	8.0
PPACA Income Tax Deferral [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		4.8	5.0
Other [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		3.8	3.6
Regulatory Clause Revenues, under-recovered [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets	[10]	$ 24.7	$ 0.0
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 was refunded to Minnesota Power’s regulated retail customers in the second quarter of 2019.

[3] Provision for Tax Reform Refund related to the income tax benefits of the TCJA in 2018 was refunded to Minnesota Power customers in the first quarter of 2019 and is being returned to SWL&P customers through 2020.

[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.

[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] Non-legal plant removal obligations included in retail customer rates that have not yet been incurred.

[7] 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 12. Pension and Other Postretirement Benefit Plans.)

[8] 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.

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

[10] The cost recovery rider regulatory assets and liabilities are revenue not yet collected from our customers and cash collections from our customers in excess of the revenue recognized, respectively, 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 assets as of December 31, 2019, will be recovered within the next two years.

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

[12] 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.

[13] 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.