Regulatory Matters - Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 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		$ 16.8	$ 10.6
Non-Current Regulatory Assets		358.9	372.0
Total Regulatory Assets		375.7	382.6
Non-Current Regulatory Liabilities		122.9	105.0
Wholesale and Retail Contra AFUDC [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities	[1]	56.9	58.0
North Dakota Investment Tax Credits [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities	[2]	27.9	12.8
Income Taxes [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities	[3]	20.0	6.1
Plant Removal Obligations [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		15.6	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
Deferred Fuel Adjustment Clause [Member]
Regulatory Assets and Liabilities [Line Items]
Current Regulatory Assets	[5]	16.8	10.6
Defined Benefit Pension and Other Postretirement Benefit Plans [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets	[4]	213.9	219.3
Income Taxes [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets	[3]	65.1	64.2
Cost Recovery Riders [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets	[6]	39.9	58.0
Asset Retirement Obligations [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets	[7]	24.8	21.6
PPACA Income Tax Deferral [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		5.0	5.0
Other
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		10.2	3.9
Non-Current Regulatory Liabilities		$ 2.5	$ 5.1

[1] Wholesale and Retail Contra AFUDC represents the regulatory offset to 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 lives of the related assets.

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

[3] These assets and liabilities are offsets to deferred income taxes recognized on certain regulatory temporary differences, which will reverse over the remaining lives of those temporary differences.

[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 12. Pension and Other Postretirement Benefit Plans.)

[5] Current regulatory assets are included in 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 the Bison Wind Energy Center, 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 September 30, 2016, will be recovered over the next two years.

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