Operations and Significant Accounting Policies (Details) - USD ($) $ in Millions		Jun. 30, 2016	Dec. 31, 2015
Inventories [Abstract]
Fuel	[1]	$ 49.2	$ 58.1
Materials and Supplies		49.8	49.1
Raw Materials		2.8	2.7
Work in Progress		0.6	0.0
Finished Goods		8.3	7.5
Reserve for Obsolescence		(0.3)	(0.3)
Total Inventories		110.4	117.1
Prepayments and Other Current Assets [Abstract]
Deferred Fuel Adjustment Clause		14.5	10.6
Restricted Cash	[2]	7.5	5.6
Other		16.4	19.5
Total Prepayments and Other Current Assets		38.4	35.7
Other Non-Current Assets [Abstract]
Restricted Cash		8.2	8.1
Contract Payment		31.0
Other Current Liabilites [Abstract]
Customer Deposits		13.4	15.1
Power Purchase Agreements		23.9	23.3
Other		48.6	47.7
Total Other Current Liabilities		85.9	86.1
Other Non-Current Liabilities [Abstract]
Asset Retirement Obligation		135.2	131.4
Power Purchase Agreements		125.9	138.1
Contingent Consideration	[3]	37.3	36.6
Other		42.4	42.9
Total Other Non-Current Liabilities		$ 340.8	$ 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] 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.)