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Regulatory Matters - Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions | 3 Months Ended |
--- | --- | ---
Mar. 31, 2018 | Dec. 31, 2017
Regulatory Assets and Liabilities [Line Items] |  |  |
Regulatory Assets and Liabilities Currently Earning a Return |  | No regulatory assets or liabilities are currently earning a return. |
Non-Current Regulatory Assets |  | $ 376.0 | $ 384.7
Current Regulatory Liabilities | [1] | 35.6 | 0.0
Non-Current Regulatory Liabilities |  | 516.0 | 532.0
Total Regulatory Liabilities |  | 551.6 | 532.0
Minnesota Power [Member] | Retail Customers [Member] | Electric Rates [Member] | Energy-Intensive Trade-Exposed Customer Rates [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Provided Discounts to EITE Customers that will be Offset Against Interim Rate Refunds |  | 12.9 |
MPUC [Member] | Minnesota Power [Member] | Retail Customers [Member] | Electric Rates [Member] | Energy-Intensive Trade-Exposed Customer Rates [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Provided Discounts to EITE Customers that will be Offset Against Interim Rate Refunds |  |  | 8.6
Provision for Interim Rate Refund [Domain] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Current Regulatory Liabilities | [1],[2] | 28.1 | 0.0
Provision for Tax Reform Refund [Domain] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Current Regulatory Liabilities | [1],[3] | 7.5 | 0.0
Income Taxes [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Liabilities |  | 406.2 | 411.2
Wholesale and Retail Contra AFUDC [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Liabilities |  | 59.1 | 57.9
Plant Removal Obligations [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Liabilities |  | 21.8 | 20.3
North Dakota Investment Tax Credits [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Liabilities |  | 14.3 | 14.1
Cost Recovery Riders [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Liabilities |  | 11.1 | 2.2
Provision for Interim Rate Refund [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Liabilities | [2] | 0.0 | 23.7
Other |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Liabilities |  | 3.5 | 2.6
Defined Benefit Pension and Other Postretirement Benefit Plans [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Assets |  | 217.7 | 220.3
Income Taxes [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Assets |  | 110.4 | 112.8
Asset Retirement Obligations [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Assets |  | 30.4 | 29.6
Manufactured Gas Plant [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Assets |  | 7.8 | 8.1
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 |  | 0.2 | 3.3
Other |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Assets |  | $ 4.5 | $ 5.6
[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 the first quarter of 2019 and includes $12.9 million of discounts provided to EITE customers that will be offset against interim rate refunds as of March 31, 2018 ($8.6 million as of December 31, 2017). (See 2016 Minnesota General Rate Case and Energy-Intensive Trade‑Exposed Customer Rates.)
[3] | We have recorded the impact of the federal income tax rate change in 2018 due to the TCJA for Minnesota Power and SWL&P as regulatory liabilities and a reduction in revenue as the benefits of the TCJA are deferred pending the outcome of regulatory proceedings with the MPUC and PSCW. (See Tax Cuts and Jobs Act of 2017.)

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