Consolidated Statement of Equity - USD ($) $ in Millions	Total	Common Stock [Member]	Accumulated Other Comprehensive Loss [Member]	Accumulated Other Comprehensive Loss [Member] Cumulative Effect, Period of Adoption, Adjustment	Retained Earnings [Member]	Retained Earnings [Member] Cumulative Effect, Period of Adoption, Adjustment	Non-Controlling Interest in Subsidiaries [Member]
Balance, Beginning of Period at Dec. 31, 2017		$ 1,401.4	$ (22.6)		$ 689.4		$ 0.0
Consolidated Statement of Equity [Roll Forward]
Common Stock Issued		27.1
Unrealized Gain (Loss) on Securities	$ (0.1)		(0.1)
Defined Benefit Pension and Other Postretirement Plans	1.0		1.0	$ 5.6
Net Loss Attributable to Non-Controlling Interest	0.0						0.0
Net Income Attributable to ALLETE	174.1				174.1	$ 6.1
Common Stock Dividends	115.0				(115.0)
Proceeds from Non-Controlling Interest - Net of Issuance Costs	0.0						0.0
Balance, Ending of Period at Dec. 31, 2018	$ 2,155.8	1,428.5	(27.3)		754.6		0.0
Consolidated Statement of Equity [Roll Forward]
Dividends Per Share of Common Stock	$ 2.24
Common Stock Issued		8.2
Unrealized Gain (Loss) on Securities	$ 0.2		0.2
Defined Benefit Pension and Other Postretirement Plans	3.5		3.5	0.0
Net Loss Attributable to Non-Controlling Interest	(0.1)						(0.1)
Net Income Attributable to ALLETE	185.6				185.6	0.0
Common Stock Dividends	121.4				(121.4)
Proceeds from Non-Controlling Interest - Net of Issuance Costs	103.8						103.8
Balance, Ending of Period at Dec. 31, 2019	$ 2,335.6	1,436.7	(23.6)		818.8		103.7
Consolidated Statement of Equity [Roll Forward]
Dividends Per Share of Common Stock	$ 2.35
Common Stock Issued		24.2
Unrealized Gain (Loss) on Securities	$ 0.1		0.1
Defined Benefit Pension and Other Postretirement Plans	(7.6)		(7.6)	$ 0.0
Net Loss Attributable to Non-Controlling Interest	(12.6)						(12.6)
Net Income Attributable to ALLETE	174.2				174.2	$ 0.0
Common Stock Dividends	128.2				(128.2)
Proceeds from Non-Controlling Interest - Net of Issuance Costs	414.5						414.5
Balance, Ending of Period at Dec. 31, 2020	$ 2,800.2	$ 1,460.9	$ (31.1)		$ 864.8		$ 505.6
Consolidated Statement of Equity [Roll Forward]
Dividends Per Share of Common Stock	$ 2.47