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Regulatory Matters - Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions | 9 Months Ended |
--- | --- | ---
Sep. 30, 2017 | Dec. 31, 2016
Regulatory Assets and Liabilities [Line Items] |  |  |
Regulatory Assets and Liabilities Currently Earning a Return |  | No regulatory assets or liabilities are currently earning a return. |
Current Regulatory Assets |  | $ 20.2 | $ 18.6
Non-Current Regulatory Assets |  | 310.6 | 330.1
Total Regulatory Assets |  | 330.8 | 348.7
Non-Current Regulatory Liabilities |  | 111.5 | 125.8
Wholesale and Retail Contra AFUDC [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Liabilities |  | 57.0 | 56.8
Plant Removal Obligations [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Liabilities |  | 19.2 | 19.1
Income Taxes [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Liabilities |  | 18.6 | 19.1
North Dakota Investment Tax Credits [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Liabilities |  | 13.9 | 28.2
Deferred Fuel Adjustment Clause [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Current Regulatory Assets |  | 20.2 | 18.6
Defined Benefit Pension and Other Postretirement Benefit Plans [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Assets |  | 221.4 | 226.1
Income Taxes [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Assets | [1] | 35.7 | 33.8
Asset Retirement Obligations [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Assets |  | 29.8 | 26.0
Cost Recovery Riders [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Assets |  | 5.2 | 30.5
PPACA Income Tax Deferral [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Assets |  | 5.0 | 5.0
Conservation Improvement Program [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Assets |  | 4.9 | 4.0
Other |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Assets |  | 8.6 | 4.7
Non-Current Regulatory Liabilities |  | $ 2.8 | $ 2.6
[1] | See Note 1. Operations and Significant Accounting Policies – Revision of Prior Balance Sheet.

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- Definition Regulatory Assets and Liabilities [Line Items] + References No definition available. + Details
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- Definition The amount for the individual regulatory asset as itemized in a table of regulatory assets as of the end of the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 + Details
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- Definition Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are expected to be recovered through revenue sources within one year or the normal operating cycle, if longer. Such costs are capitalized if they meet both of the following criteria: a. It is probable that future revenue in an amount at least equal to the capitalized cost will result from inclusion of that cost in allowable costs for rate-making purposes. b. Based on available evidence, the future revenue will be provided to permit recovery of the previously incurred cost rather than to provide for expected levels of similar future costs. If the revenue will be provided through an automatic rate-adjustment clause, this criterion requires that the regulator's intent clearly be to permit recovery of the previously incurred cost. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 + Details
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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 + Details
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- Definition The amount for the individual regulatory noncurrent liability as itemized in a table of regulatory noncurrent liabilities as of the end of the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.24) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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