Consolidated Statement of Shareholders' 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 Shareholders' Equity [Roll Forward]
Common Stock Issued		4,800,000
Unrealized Gain (Loss) on Debt Securities	$ 200,000		200,000
Defined Benefit Pension and Other Postretirement Plans	200,000		200,000
Net Income (Loss) Attributable to ALLETE	104,700,000			104,700,000
Common Stock Dividends	60,700,000			(60,700,000)
Proceeds from Noncontrolling Interests	0				0
Net Loss Attributable to Non-Controlling Interest	0				0
Balance, End of Period at Jun. 30, 2019	$ 2,205,000,000.0	1,433,300,000	(26,900,000)	798,600,000	0
Consolidated Statement of Shareholders' Equity [Roll Forward]
Dividends Per Share of Common Stock	$ 1.175
Balance, Beginning Balance at Mar. 31, 2019		1,431,100,000	(27,200,000)	794,800,000	0
Consolidated Statement of Shareholders' Equity [Roll Forward]
Common Stock Issued		2,200,000
Unrealized Gain (Loss) on Debt Securities	$ 100,000		100,000
Defined Benefit Pension and Other Postretirement Plans	200,000		200,000
Net Income (Loss) Attributable to ALLETE	34,200,000			34,200,000
Common Stock Dividends				(30,400,000)
Proceeds from Noncontrolling Interests					0
Net Loss Attributable to Non-Controlling Interest	0				0
Balance, End of Period at Jun. 30, 2019	$ 2,205,000,000.0	1,433,300,000	(26,900,000)	798,600,000	0
Consolidated Statement of Shareholders' 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 Shareholders' Equity [Roll Forward]
Common Stock Issued		11,000,000.0
Unrealized Gain (Loss) on Debt Securities	0		0
Defined Benefit Pension and Other Postretirement Plans	300,000		300,000
Net Income (Loss) Attributable to ALLETE	86,400,000			86,400,000
Common Stock Dividends	63,900,000			(63,900,000)
Proceeds from Noncontrolling Interests	67,800,000				67,800,000
Net Loss Attributable to Non-Controlling Interest	(5,000,000.0)				(5,000,000.0)
Balance, End of Period at Jun. 30, 2020	$ 2,432,200,000	1,447,700,000	(23,300,000)	841,300,000	166,500,000
Consolidated Statement of Shareholders' Equity [Roll Forward]
Dividends Per Share of Common Stock	$ 1.235
Balance, Beginning Balance at Mar. 31, 2020		1,441,700,000	(23,800,000)	853,200,000	101,900,000
Consolidated Statement of Shareholders' Equity [Roll Forward]
Common Stock Issued		6,000,000.0
Unrealized Gain (Loss) on Debt Securities	$ 400,000		400,000
Defined Benefit Pension and Other Postretirement Plans	100,000		100,000
Net Income (Loss) Attributable to ALLETE	20,100,000			20,100,000
Common Stock Dividends				(32,000,000.0)
Proceeds from Noncontrolling Interests					67,800,000
Net Loss Attributable to Non-Controlling Interest	(3,200,000)				(3,200,000)
Balance, End of Period at Jun. 30, 2020	$ 2,432,200,000	$ 1,447,700,000	$ (23,300,000)	$ 841,300,000	$ 166,500,000
Consolidated Statement of Shareholders' Equity [Roll Forward]
Dividends Per Share of Common Stock	$ 0.6175