Goodwill and Intangible Assets - Goodwill (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016		Dec. 31, 2015	Dec. 31, 2014
Goodwill [Roll Forward]
Beginning Balance		$ 130.6		$ 2.9
Acquired Goodwill	[1]	3.9		127.7
Impairment Charge	[2]	(3.3)	[3]	0.0	$ 0.0
Ending Balance		131.2		130.6	2.9
ALLETE Clean Energy [Member]
Goodwill [Roll Forward]
Beginning Balance		3.3		2.9
Acquired Goodwill	[1]	0.0		0.4
Impairment Charge		(3.3)	[3]	0.0	0.0
Ending Balance		0.0		3.3	2.9
U.S. Water Services [Member]
Goodwill [Roll Forward]
Beginning Balance		127.3		0.0
Acquired Goodwill	[1]	3.9		127.3
Impairment Charge		0.0	[3]	0.0
Ending Balance		$ 131.2		$ 127.3	$ 0.0

[1] See Note 6. Acquisitions.

[2] See Goodwill and Intangible Assets.

[3] The facts and circumstances that led to an impairment of ALLETE Clean Energy’s goodwill primarily relate to lower estimated energy prices in periods not under PSAs. Impairment Charge is included in Operating Expenses – Other on the Consolidated Statement of Income. (See Note 1. Operations and Significant Accounting Policies.) ALLETE Clean Energy’s goodwill was primarily related to the acquisition of Storm Lake II in January 2014.