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Regulatory Matters - Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions | 6 Months Ended |
--- | --- | ---
Jun. 30, 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 |  | $ 374.5 | $ 384.7
Current Regulatory Liabilities | [1] | 39.2 | 0.0
Non-Current Regulatory Liabilities |  | 516.0 | 532.0
Total Regulatory Liabilities |  | 555.2 | 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 |  | 16.7 |
Minnesota Power [Member] | Retail Customers [Member] | Electric Rates [Member] | Energy-Intensive Trade-Exposed Customer Rates [Member] | MPUC [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] | 32.5 | 0.0
Provision for Tax Reform Refund [Domain] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Current Regulatory Liabilities | [1],[3] | 6.7 | 0.0
Income Taxes [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Liabilities |  | 401.4 | 411.2
Wholesale and Retail Contra AFUDC [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Liabilities |  | 60.2 | 57.9
Plant Removal Obligations [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Liabilities |  | 23.0 | 20.3
Cost Recovery Riders [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Liabilities |  | 16.8 | 2.2
North Dakota Investment Tax Credits [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Liabilities |  | 14.4 | 14.1
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 |  | 0.2 | 2.6
Defined Benefit Pension and Other Postretirement Benefit Plans [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Assets |  | 216.7 | 220.3
Income Taxes [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Assets |  | 109.3 | 112.8
Asset Retirement Obligations [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Assets |  | 31.0 | 29.6
Manufactured Gas Plant [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Assets |  | 8.0 | 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 |  | 0.0 | 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 $16.7 million of discounts provided to EITE customers that will be offset against interim rate refunds as of June 30, 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 income tax changes for Minnesota Power and SWL&P resulting from the TCJA in 2018 as regulatory liabilities and a reduction in revenue as the benefits 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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