Consolidated Statement of Equity - USD ($) $ in Millions	Total	Common Stock [Member]	Accumulated Other Comprehensive Loss [Member]	Retained Earnings [Member]	Non-controlling Interest [Member]
Balance, Beginning Balance at Dec. 31, 2020		$ 1,460.9	$ (31.1)	$ 864.8	$ 505.6
Consolidated Statement of Equity [Roll Forward]
Common Stock Issued		35.6
Unrealized Loss on Securities	$ 0.0		0.0
Defined Benefit Pension and Other Postretirement Plans	1.2		1.2
Net Income Attributable ALLETE	107.3			107.3
Common Stock Dividends	98.7			(98.7)
Proceeds from Non-controlling Interest in Subsidiaries - Net of Issuance Costs	28.9				28.9
Net Loss Attributable to Non-Controlling Interest	(18.1)				(18.1)
Distributions to Non-controlling Interest					(1.9)
Balance, End of Period at Sep. 30, 2021	$ 2,854.5	1,496.5	(29.9)	873.4	514.5
Consolidated Statement of Equity [Roll Forward]
Dividends Per Share of Common Stock	$ 1.89
Balance, Beginning Balance at Jun. 30, 2021		1,474.1	(30.3)	878.8	519.3
Consolidated Statement of Equity [Roll Forward]
Common Stock Issued		22.4
Unrealized Loss on Securities	$ 0.0		0.0
Defined Benefit Pension and Other Postretirement Plans	0.4		0.4
Net Income Attributable ALLETE	27.6			27.6
Common Stock Dividends				(33.0)
Proceeds from Non-controlling Interest in Subsidiaries - Net of Issuance Costs					0.0
Net Loss Attributable to Non-Controlling Interest	(3.5)				(3.5)
Distributions to Non-controlling Interest					(1.3)
Balance, End of Period at Sep. 30, 2021	$ 2,854.5	1,496.5	(29.9)	873.4	514.5
Consolidated Statement of Equity [Roll Forward]
Dividends Per Share of Common Stock	$ 0.63
Balance, Beginning Balance at Dec. 31, 2021	$ 2,946.3	1,536.7	(23.8)	900.2	533.2
Consolidated Statement of Equity [Roll Forward]
Common Stock Issued		240.5
Unrealized Loss on Securities	(0.4)		(0.4)
Defined Benefit Pension and Other Postretirement Plans	0.4		0.4
Net Income Attributable ALLETE	137.6			137.6
Common Stock Dividends	108.6			(108.6)
Proceeds from Non-controlling Interest in Subsidiaries - Net of Issuance Costs	155.7				182.9
Net Loss Attributable to Non-Controlling Interest	(43.5)				(43.5)
Distributions to Non-controlling Interest					(1.3)
Balance, End of Period at Sep. 30, 2022	$ 3,353.9	1,777.2	(23.8)	929.2	671.3
Consolidated Statement of Equity [Roll Forward]
Dividends Per Share of Common Stock	$ 1.95
Balance, Beginning Balance at Jun. 30, 2022		1,771.7	(23.9)	932.6	678.5
Consolidated Statement of Equity [Roll Forward]
Common Stock Issued		5.5
Unrealized Loss on Securities	$ 0.0		0.0
Defined Benefit Pension and Other Postretirement Plans	0.1		0.1
Net Income Attributable ALLETE	33.7			33.7
Common Stock Dividends				(37.1)
Proceeds from Non-controlling Interest in Subsidiaries - Net of Issuance Costs					0.0
Net Loss Attributable to Non-Controlling Interest	(6.9)				(6.9)
Distributions to Non-controlling Interest					(0.3)
Balance, End of Period at Sep. 30, 2022	$ 3,353.9	$ 1,777.2	$ (23.8)	$ 929.2	$ 671.3
Consolidated Statement of Equity [Roll Forward]
Dividends Per Share of Common Stock	$ 0.65