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Regulatory Matters - Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2021	Dec. 31, 2020
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		$ 472.7	$ 480.9
Current Regulatory Liabilities	[1]	4.6	7.6
Non-Current Regulatory Liabilities		517.9	524.8
Total Regulatory Liabilities		522.5	532.4
Regulatory Clause Revenues, under-recovered
Regulatory Assets and Liabilities [Line Items]
Current Regulatory Liabilities	[1]	1.8	3.7
Transmission Formula Rates [Member]
Regulatory Assets and Liabilities [Line Items]
Current Regulatory Liabilities	[1]	2.4	2.9
Income Taxes [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		367.8	375.3
Wholesale and Retail Contra AFUDC [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		85.9	86.6
Plant Removal Obligations [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		43.0	41.2
Pension and Other Postretirement Plan Costs [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		3.4	4.4
North Dakota Investment Tax Credits [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		12.4	12.0
Conservation Improvement Program [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		1.7	1.5
Other
Regulatory Assets and Liabilities [Line Items]
Current Regulatory Liabilities	[1]	0.4	1.0
Non-Current Regulatory Liabilities		3.7	3.8
Pension and Other Postretirement Plan Costs [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		255.3	259.7
Income Taxes [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		110.7	113.7
Cost Recovery Riders [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		57.5	54.0
Asset Retirement Obligations [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		31.5	31.6
Manufactured Gas Plant [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		5.8	8.8
Boswell Units 1 and 2 Net Plant and Equipment [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		3.6	5.0
PPACA Income Tax Deferral [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		4.5	4.5
Other
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		$ 3.8	$ 3.6
[1] Current regulatory liabilities are presented within Other Current Liabilities on the Consolidated Balance Sheet

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Regulatory Assets and Liabilities [Line Items]

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Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are not expected to be recovered through revenue sources within one year or the normal operating cycle if longer.

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The amount for the individual regulatory liability as itemized in a table of regulatory liabilities as of the end of the period.

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Identifies what net regulatory assets are earning a return through rate billings.

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