Goodwill and Intangible Assets - Intangible Assets (Details) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2019	Sep. 30, 2018
Definite-Lived Intangible Assets [Roll Forward]
Beginning Balance		$ 58.2
Amortization		(1.5)	$ (4.1)
Other Changes	[1]	(55.7)
Ending Balance		1.0
Intangible Assets [Abstract]
Total Intangible Assets - Beginning Balance		74.8
Total Intangible Assets - Amortization		(1.5)	$ (4.1)
Total Intangible Assets - Other Changes	[1]	(72.3)
Total Intangible Assets - Ending Balance		1.0
Trademarks and Trade Names [Member]
Indefinite-Lived Intangible Assets [Roll Forward]
Beginning Balance		16.6
Other Changes	[1]	(16.6)
Ending Balance		0.0
Customer Relationships [Member]
Definite-Lived Intangible Assets [Roll Forward]
Beginning Balance		50.7
Amortization		(1.1)
Other Changes	[1]	(49.6)
Ending Balance		0.0
Intangible Assets [Abstract]
Total Intangible Assets - Amortization		(1.1)
Developed Technology and Other [Member]
Definite-Lived Intangible Assets [Roll Forward]
Beginning Balance	[2]	7.5
Amortization	[2]	(0.4)
Other Changes	[1],[2]	(6.1)
Ending Balance	[2]	1.0
Intangible Assets [Abstract]
Total Intangible Assets - Amortization	[2]	$ (0.4)

[1] On March 26, 2019, ALLETE completed the sale of U.S. Water Services which resulted in the removal of the related intangible assets from the Consolidated Balance Sheet.

[2] Developed Technology and Other includes patents, non-compete agreements, land easements and trade names with finite lives.