Consolidated Statement of Income - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating Revenue [Abstract]
Contracts with Customers – Utility	$ 1,042.4	$ 1,059.5	$ 1,063.8
Contracts with Customers – Non-utility	186.5	415.5	331.9
Other – Non-utility	11.6	23.6	23.6
Total Operating Revenue	1,240.5	1,498.6	1,419.3
Operating Expenses [Abstract]
Fuel, Purchased Power and Gas - Utility	390.7	407.5	396.9
Transmission Services - Utility	69.8	69.9	71.2
Cost of Sales - Non-utility	80.6	218.0	147.5
Operating and Maintenance	264.3	340.5	344.1
Depreciation and Amortization	202.0	205.6	177.5
Taxes Other Than Income Taxes	53.3	57.9	56.9
Operating Expenses – Other	0.0	(2.0)	(0.7)
Total Operating Expenses	1,060.7	1,297.4	1,193.4
Operating Income	179.8	201.2	225.9
Other Income (Expense) [Abstract]
Interest Expense	(64.9)	(67.9)	(67.8)
Equity Earnings	21.7	17.5	22.5
Gain on Sale of U.S. Water Services	23.6	0.0	0.0
Other	18.7	7.8	6.3
Total Other Expense	(0.9)	(42.6)	(39.0)
Income Before Non-Controlling Interest and Income Taxes	178.9	158.6	186.9
Income Tax Expense	(6.6)	(15.5)	14.7
Net Income	185.5	174.1	172.2
Less: Non-Controlling Interest in Subsidiaries	(0.1)	0.0	0.0
Net Income Attributable to ALLETE	$ 185.6	$ 174.1	$ 172.2
Average Shares of Common Stock and Per Share Data [Abstract]
Basic (Shares)	51.6	51.3	50.8
Diluted (Shares)	51.7	51.5	51.0
Basic Earnings Per Share of Common Stock	$ 3.59	$ 3.39	$ 3.39
Diluted Earnings Per Share of Common Stock	$ 3.59	$ 3.38	$ 3.38