Consolidated Statement of Equity - USD ($)	Total	Common Stock [Member]	AOCI Attributable to Parent [Member]	Retained Earnings [Member]	Noncontrolling Interest [Member]
Balance, Beginning Balance at Dec. 31, 2018		$ 1,428,500,000	$ (27,300,000)	$ 754,600,000	$ 0
Consolidated Statement of Equity [Roll Forward]
Common Stock Issued		6,600,000
Unrealized Gain (Loss) on Debt Securities	$ 200,000		200,000
Defined Benefit Pension and Other Postretirement Plans	300,000		300,000
Net Income (Loss) Attributable to ALLETE	135,900,000			135,900,000
Common Stock Dividends	91,000,000.0			(91,000,000.0)
Proceeds from Noncontrolling Interests	0				0
Net Loss Attributable to Non-Controlling Interest	0				0
Balance, End of Period at Sep. 30, 2019	$ 2,207,800,000	1,435,100,000	(26,800,000)	799,500,000	0
Consolidated Statement of Equity [Roll Forward]
Dividends Per Share of Common Stock	$ 1.7625
Balance, Beginning Balance at Jun. 30, 2019		1,433,300,000	(26,900,000)	798,600,000	0
Consolidated Statement of Equity [Roll Forward]
Common Stock Issued		1,800,000
Unrealized Gain (Loss) on Debt Securities	$ 0		0
Defined Benefit Pension and Other Postretirement Plans	100,000		100,000
Net Income (Loss) Attributable to ALLETE	31,200,000			31,200,000
Common Stock Dividends				(30,300,000)
Proceeds from Noncontrolling Interests					0
Net Loss Attributable to Non-Controlling Interest	0				0
Balance, End of Period at Sep. 30, 2019	$ 2,207,800,000	1,435,100,000	(26,800,000)	799,500,000	0
Consolidated Statement of Equity [Roll Forward]
Dividends Per Share of Common Stock	$ 0.5875
Balance, Beginning Balance at Dec. 31, 2019	$ 2,335,600,000	1,436,700,000	(23,600,000)	818,800,000	103,700,000
Consolidated Statement of Equity [Roll Forward]
Common Stock Issued		17,500,000
Unrealized Gain (Loss) on Debt Securities	0		0
Defined Benefit Pension and Other Postretirement Plans	500,000		500,000
Net Income (Loss) Attributable to ALLETE	127,100,000			127,100,000
Common Stock Dividends	96,000,000.0			(96,000,000.0)
Proceeds from Noncontrolling Interests	67,800,000				67,800,000
Net Loss Attributable to Non-Controlling Interest	(6,900,000)				(6,900,000)
Balance, End of Period at Sep. 30, 2020	$ 2,445,600,000	1,454,200,000	(23,100,000)	849,900,000	164,600,000
Consolidated Statement of Equity [Roll Forward]
Dividends Per Share of Common Stock	$ 1.8525
Balance, Beginning Balance at Jun. 30, 2020		1,447,700,000	(23,300,000)	841,300,000	166,500,000
Consolidated Statement of Equity [Roll Forward]
Common Stock Issued		6,500,000
Unrealized Gain (Loss) on Debt Securities	$ 0		0
Defined Benefit Pension and Other Postretirement Plans	200,000		200,000
Net Income (Loss) Attributable to ALLETE	40,700,000			40,700,000
Common Stock Dividends				(32,100,000)
Proceeds from Noncontrolling Interests					0
Net Loss Attributable to Non-Controlling Interest	(1,900,000)				(1,900,000)
Balance, End of Period at Sep. 30, 2020	$ 2,445,600,000	$ 1,454,200,000	$ (23,100,000)	$ 849,900,000	$ 164,600,000
Consolidated Statement of Equity [Roll Forward]
Dividends Per Share of Common Stock	$ 0.6175