Consolidated Balance Sheet Parentheticals - USD ($) shares in Millions, $ in Millions		Dec. 31, 2019	Dec. 31, 2018
Accounts Receivable [Abstract]
Accounts Receivable, Allowance for Credit Loss, Current	[1]	$ 0.9	$ 1.7
Common Stock [Abstract]
Common Stock, Par Value Per Share		$ 0.00	$ 0.00
Common Stock, Shares Authorized		80.0	80.0
Common Stock, Shares Outstanding		51.7	51.5
Common Stock, Shares Issued		51.7	51.5

[1] On March 26, 2019, ALLETE sold U.S. Water Services which resulted in the removal of the related accounts receivable from the Consolidated Balance Sheet.