Regulatory Matters - Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015
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		$ 18.6	$ 10.6
Non-Current Regulatory Assets		359.6	372.0
Total Regulatory Assets		378.2	382.6
Non-Current Regulatory Liabilities		125.8	105.0
Wholesale and Retail Contra AFUDC [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities	[1]	56.8	58.0
North Dakota Investment Tax Credits [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities	[2]	28.2	12.8
Income Taxes [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities	[3]	19.1	6.1
Plant Removal Obligations [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		19.1	22.1
Defined Benefit Pension and Other Postretirement Benefit Plans [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities	[4]	0.0	0.9
Other [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		2.6	5.1
Deferred Fuel Adjustment Clause [Member]
Regulatory Assets and Liabilities [Line Items]
Current Regulatory Assets	[5]	18.6	10.6
Defined Benefit Pension and Other Postretirement Benefit Plans [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets	[4]	226.1	219.3
Income Taxes [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets	[3]	63.3	64.2
Cost Recovery Riders [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets	[6]	$ 30.5	58.0
Alternative Revenue Program, Required Collection Period (Years)		2 years
Asset Retirement Obligation [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets	[7]	$ 26.0	21.6
PPACA Income Tax Deferral [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		5.0	5.0
Other [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		$ 8.7	$ 3.9

[1] Wholesale and Retail Contra AFUDC represents amortization to offset AFUDC Equity and Debt recorded during the construction period of our cost recovery rider projects prior to placing the projects in service. The regulatory liability will decrease over the remaining depreciable life of the related asset.

[2] North Dakota investment tax credits expected to be realized from Bison that will be credited to Minnesota Power’s regulated retail customers over the remaining life of Bison through future renewable cost recovery rider fillings.

[3] These costs represent the difference between deferred income taxes recognized for financial reporting purposes and amounts previously billed to our customers. This balance will decrease over the remaining life of the related temporary differences and flow through current income taxes.

[4] Defined benefit pension and other postretirement items included in our Regulated Operations, which are otherwise required to be recognized in accumulated other comprehensive income as actuarial gains and losses as well as prior service costs and credits, are recognized as regulatory assets or regulatory liabilities on the Consolidated Balance Sheet. The asset or liability will decrease as the deferred items are amortized and recognized as components of net periodic benefit cost. (See Note 15. Pension and Other Postretirement Benefit Plans.)

[5] Current regulatory assets are presented within Prepayments and Other on the Consolidated Balance Sheet.

[6] The cost recovery rider regulatory assets are revenues not yet collected from our customers primarily due to capital expenditures related to Bison, investment in CapX2020 projects, and the Boswell Unit 4 environmental upgrade and are recognized in accordance with the accounting standards for alternative revenue programs. The cost recovery rider regulatory assets as of December 31, 2016, will be recovered within the next two years.

[7] Asset retirement obligations will accrete and be amortized over the lives of the related property with asset retirement obligations.