Consolidated Statement of Equity - USD ($) $ in Millions	Total	Common Stock [Member]	Accumulated Other Comprehensive Loss [Member]	Retained Earnings [Member]	Non-Controlling Interest in Subsidiaries [Member]
Balance, Beginning of Period at Dec. 31, 2019		$ 1,436.7	$ (23.6)	$ 818.8	$ 103.7
Consolidated Statement of Equity [Roll Forward]
Common Stock Issued		24.2
Unrealized Gain (Loss) on Debt Securities	$ 0.1		0.1
Defined Benefit Pension and Other Postretirement Plans	(7.6)		(7.6)
Net Loss Attributable to Non-Controlling Interest	(12.6)				(12.6)
Reclassification of Redeemable Non-Controlling Interest to Current Liabilities					0.0
Distributions to Non-Controlling Interest					0.0
Net Income Attributable to ALLETE	165.4			165.4
Common Stock Dividends	128.2			(128.2)
Adjustment of Redeemable Non-Controlling Interest				0.0
Proceeds from Non-Controlling Interest - Net of Issuance Costs	414.5				414.5
Balance, Ending of Period at Dec. 31, 2020	$ 2,791.4	1,460.9	(31.1)	856.0	505.6
Consolidated Statement of Equity [Roll Forward]
Dividends Per Share of Common Stock	$ 2.47
Common Stock Issued		75.8
Unrealized Gain (Loss) on Debt Securities	$ (0.1)		(0.1)
Defined Benefit Pension and Other Postretirement Plans	7.4		7.4
Net Loss Attributable to Non-Controlling Interest	(31.4)				(31.4)
Reclassification of Redeemable Non-Controlling Interest to Current Liabilities					28.8
Distributions to Non-Controlling Interest					(3.1)
Net Income Attributable to ALLETE	169.2			169.2
Common Stock Dividends	131.9			(131.9)
Adjustment of Redeemable Non-Controlling Interest				(1.9)
Proceeds from Non-Controlling Interest - Net of Issuance Costs	90.9				90.9
Balance, Ending of Period at Dec. 31, 2021	$ 2,937.5	1,536.7	(23.8)	891.4	533.2
Consolidated Statement of Equity [Roll Forward]
Dividends Per Share of Common Stock	$ 2.52
Common Stock Issued		244.8
Unrealized Gain (Loss) on Debt Securities	$ (0.4)		(0.4)
Defined Benefit Pension and Other Postretirement Plans	(0.2)		(0.2)
Net Loss Attributable to Non-Controlling Interest	(58.0)				(58.0)
Reclassification of Redeemable Non-Controlling Interest to Current Liabilities					0.0
Distributions to Non-Controlling Interest					(1.7)
Net Income Attributable to ALLETE	189.3			189.3
Common Stock Dividends	145.9			(145.9)
Adjustment of Redeemable Non-Controlling Interest				0.0
Proceeds from Non-Controlling Interest - Net of Issuance Costs	155.7				182.9
Balance, Ending of Period at Dec. 31, 2022	$ 3,348.3	$ 1,781.5	$ (24.4)	$ 934.8	$ 656.4
Consolidated Statement of Equity [Roll Forward]
Dividends Per Share of Common Stock	$ 2.60