Consolidated Statement of Income - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating Revenue [Abstract]
Contracts with Customers – Utility	$ 1,059.5	$ 1,063.8	$ 1,000.7
Contracts with Customers – Non-utility	415.5	331.9	316.7
Other – Non-utility	23.6	23.6	22.3
Total Operating Revenue	1,498.6	1,419.3	1,339.7
Operating Expenses [Abstract]
Fuel, Purchased Power and Gas - Utility	407.5	396.9	339.9
Transmission Services - Utility	69.9	71.2	65.2
Cost of Sales - Non-utility	218.0	147.5	137.4
Operating and Maintenance	340.5	344.1	340.9
Depreciation and Amortization	205.6	177.5	195.8
Taxes Other Than Income Taxes	57.9	56.9	53.8
Operating Expenses – Other	(2.0)	(0.7)	(10.3)
Total Operating Expenses	1,297.4	1,193.4	1,122.7
Operating Income	201.2	225.9	217.0
Other Income (Expense) [Abstract]
Interest Expense	(67.9)	(67.8)	(70.3)
Equity Earnings in ATC	17.5	22.5	18.5
Other	7.8	6.3	10.4
Total Other Expense	(42.6)	(39.0)	(41.4)
Income Before Non-Controlling Interest and Income Taxes	158.6	186.9	175.6
Income Tax Expense	(15.5)	14.7	19.8
Net Income	174.1	172.2	155.8
Less: Non-Controlling Interest in Subsidiaries	0.0	0.0	0.5
Net Income Attributable to ALLETE	$ 174.1	$ 172.2	$ 155.3
Average Shares of Common Stock and Per Share Data [Abstract]
Basic (Shares)	51.3	50.8	49.3
Diluted (Shares)	51.5	51.0	49.5
Basic Earnings Per Share of Common Stock	$ 3.39	$ 3.39	$ 3.15
Diluted Earnings Per Share of Common Stock	$ 3.38	$ 3.38	$ 3.14