Operations and Significant Accounting Policies (Details) - USD ($) shares in Millions, $ in Millions			9 Months Ended
		Sep. 22, 2016	Sep. 30, 2016		May 23, 2016	Dec. 31, 2015
Inventories [Abstract]
Fuel	[1]		$ 45.9			$ 58.1
Materials and Supplies			49.8			49.1
Raw Materials			3.3			2.7
Work in Progress			1.1			0.0
Finished Goods			7.8			7.5
Reserve for Obsolescence			(0.5)			(0.3)
Total Inventories			107.4			117.1
Prepayments and Other Current Assets [Abstract]
Deferred Fuel Adjustment Clause			16.8			10.6
Restricted Cash	[2]		7.0			5.6
Other			15.1			19.5
Total Prepayments and Other Current Assets			38.9			35.7
Other Non-Current Assets [Abstract]
Contract Payment			30.1	[3]	$ 31.0	0.0	[3]
Finance Receivable	[4]		11.6			0.0
Restricted Cash	[5]		4.6			8.1
Other			60.2			60.0
Total Other Non-Current Assets			106.5			68.1
Real Estate Sale Consideration [Abstract]
Total Consideration for Land Inventory		$ 21.0
Down Payment of ALLETE Common Stock, Shares		0.1
Down Payment of ALLETE Common Stock, Value		$ 8.0	8.0
Other Current Liabilites [Abstract]
Customer Deposits			9.0			15.1
Power Purchase Agreements			24.3			23.3
Other			60.3			47.7
Total Other Current Liabilities			93.6			86.1
Other Non-Current Liabilities [Abstract]
Asset Retirement Obligation			136.5			131.4
Power Purchase Agreements			119.8			138.1
Contingent Consideration	[6]		37.9			36.6
Other			41.7			42.9
Total Other Non-Current Liabilities			$ 335.9			$ 349.0

[1] Fuel consists primarily of coal inventory at Minnesota Power.

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

[3] Contract Payment includes a $31.0 million payment made to Cliffs as part of a long-term power sales agreement between Minnesota Power and Silver Bay Power. The contract payment will be amortized over the term of the sales agreement. (See Note 13. Commitments, Guarantees and Contingencies.)

[4] On September 22, 2016, ALLETE Properties sold its Ormond Crossings project and Lake Swamp wetland mitigation bank for consideration of approximately $21 million. The consideration included a down payment in the form of 0.1 million shares of ALLETE common stock with a value of $8.0 million. The remaining purchase price will be paid under the terms of a finance receivable due over a five-year period which bears interest at market rates and is collateralized by the property sold.

[5] Restricted Cash includes collateral deposits required under ALLETE Clean Energy’s loan agreements and PPAs.

[6] Contingent Consideration relates to the estimated fair value of the earnings-based payment resulting from the U.S. Water Services acquisition. (See Note 3. Acquisitions and Note 5. Fair Value.)