Operations and Significant Accounting Policies - Balance Sheet and Income Statement Disclosures (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2015		Dec. 31, 2014	Dec. 31, 2013
Trade Accounts Receivable [Abstract]
Billed		$ 105.3		$ 85.5
Unbilled		16.9		18.6
Less: Allowance for Doubtful Accounts		1.0		1.1
Total Accounts Receivable		121.2		103.0
Inventories [Abstract]
Fuel	[1]	58.1		29.0
Materials and Supplies		49.1		51.5
Raw Materials		2.7		0.0
Finished Goods		7.5		0.0
Reserve for Obsolescence		(0.3)		0.0
Total Inventories		117.1		80.5
Prepayments and Other Current Assets [Abstract]
Deferred Fuel Adjustment Clause		10.6		16.3
Construction Costs for Development Project	[2]	0.0		48.2
Restricted Cash	[3]	5.6		2.7
Other Assets		19.5		14.8
Total Prepayments and Other Current Assets		35.7		82.0
Property, Plant and Equipment [Line Items]
Remaining Net Book Value		3,669.1		3,284.8
Impairment of Long-Lived Assets [Abstract]
Total Carrying Value of Real Estate Assets Before Impairment Charge		83.3
Impairment of Real Estate		36.3		0.0	$ 0.0
Other Non-Current Assets [Abstract]
Restricted Cash		8.1		5.3
Other Current Liabilities [Abstract]
Customer Deposits		15.1		19.7
Power Purchase Agreements	[4]	23.3		19.4
Construction Deposits Received for Development Project	[5]	0.0		54.3
Other		47.7	[6]	27.4
Total Other Current Liabilities		86.1		120.8
Customer Refund Liabilities		7.0
Other Non-Current Liabilities [Abstract]
Asset Retirement Obligation		131.4		109.2
Power Purchase Agreements	[7]	138.1		110.7
Contingent Consideration	[8]	36.6		0.0
Other		42.9		45.1
Total Other Non-Current Liabilities		349.0		265.0
Business Segments [Line Items]
Amortization of Power Purchase Agreements		$ 23.2		12.7	0.0
Income Taxes [Abstract]
More-Likely-Than-Not Percentage		50.00%
Current Deferred Income Tax Assets Reclassified to Non-Current Deferred Income Tax Liabilities		$ 39.4
Regulated Operations [Member]
Prepayments and Other Current Assets [Abstract]
Construction Costs for Development Project		101.2		355.4
Property, Plant and Equipment [Line Items]
Remaining Net Book Value		$ 3,114.1		2,998.5
Business Segments [Line Items]
Alternative Revenue Program, Required Collection Period		24 months
ALLETE Clean Energy [Member]
Prepayments and Other Current Assets [Abstract]
Construction Costs for Development Project		$ 4.0		1.3
Property, Plant and Equipment [Line Items]
Remaining Net Book Value		447.3		196.6
Business Segments [Line Items]
Amortization of Power Purchase Agreements		23.2		12.7
Corporate and Other [Member]
Prepayments and Other Current Assets [Abstract]
Construction Costs for Development Project		4.5		4.6
Property, Plant and Equipment [Line Items]
Remaining Net Book Value		95.5		89.7
Impairment of Long-Lived Assets [Abstract]
Impairment of Real Estate		$ 36.3		$ 0.0	$ 0.0
Business Segments [Line Items]
Minimum Cash Collections Requirement for Real Estate Sales (Percent)		20.00%
Minimum [Member]
Definite-Lived Intangible Assets [Line Items]
Useful Life (Years)		3 years
Maximum [Member]
Definite-Lived Intangible Assets [Line Items]
Useful Life (Years)		22 years
Taconite Harbor [Member]
Property, Plant and Equipment [Line Items]
Remaining Net Book Value		$ 100.0

[1] Fuel consists primarily of coal inventory at Minnesota Power. During 2015, Minnesota Power increased its coal inventory to take advantage of favorable pricing.

[2] Construction Costs for Development Project related to ALLETE Clean Energy’s project to construct a wind energy facility which was sold in 2015. (See Note 7. Acquisitions.)

[3] Restricted Cash includes collateral deposits required under ALLETE Clean Energy’s loan agreements and collateral deposits required for U.S. Water Services’ standby letters of credit.

[4] Power Purchase Agreements acquired in conjunction with the ALLETE Clean Energy wind energy facilities acquisitions. (See Note 7. Acquisitions.)

[5] Construction Deposits Received for Development Project relate to ALLETE Clean Energy’s project to construct a wind energy facility which was sold in 2015. (See Note 7. Acquisitions.)

[6] Increase in 2015 includes customer refund liabilities of approximately $7 million, pending regulatory approval.

[7] Power Purchase Agreements acquired in conjunction with the ALLETE Clean Energy wind energy facilities acquisitions. (See Note 7. Acquisitions.)

[8] Contingent Consideration relates to the estimated fair value of the earnings-based payment resulting from the U.S. Water Services acquisition. (See Note 7. Acquisitions and Note 10. Fair Value.)