Regulatory Matters - Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2019	Dec. 31, 2018
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets 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		$ 393.1	$ 389.5
Current Regulatory Liabilities	[1]	2.7	55.1
Non-Current Regulatory Liabilities		510.4	512.1
Total Regulatory Liabilities		513.1	567.2
Provision for Interim Rate Refund [Domain]
Regulatory Assets and Liabilities [Line Items]
Current Regulatory Liabilities	[1],[2]	0.0	40.0
Transmission Formula Rates [Member]
Regulatory Assets and Liabilities [Line Items]
Current Regulatory Liabilities	[1]	2.4	4.4
Non-Current Regulatory Liabilities		0.4	1.6
Provision for Tax Reform Refund [Domain]
Regulatory Assets and Liabilities [Line Items]
Current Regulatory Liabilities	[1],[3]	0.3	10.7
Income Taxes [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		382.4	396.4
Wholesale and Retail Contra AFUDC [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		74.7	64.4
Plant Removal Obligations [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		32.6	25.1
North Dakota Investment Tax Credits [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		12.3	14.7
CIP Financial Incentive [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		5.9	1.5
Cost Recovery Riders [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		0.0	6.9
Other
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		2.1	1.5
Defined Benefit Pension and Other Postretirement Benefit Plans [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		215.0	218.5
Income Taxes [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		100.9	105.5
Asset Retirement Obligations [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		32.1	32.6
Boswell 1 & 2 [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		12.1	16.3
Cost Recovery Riders [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		16.8	0.0
Manufactured Gas Plant [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		8.1	8.0
PPACA Income Tax Deferral [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		4.8	5.0
Other
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		$ 3.3	$ 3.6

[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 will be refunded to SWL&P customers in 2019 and 2020.