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		2,600,000
Unrealized Gain (Loss) on Debt Securities	$ 100,000		100,000
Defined Benefit Pension and Other Postretirement Plans	0		0
Net Income (Loss) Attributable to ALLETE	70,500,000			70,500,000
Common Stock Dividends	30,300,000			(30,300,000)
Net Loss Attributable to Non-Controlling Interest	0				0
Balance, End of Period at Mar. 31, 2019	$ 2,198,700,000	1,431,100,000	(27,200,000)	794,800,000	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		5,000,000.0
Unrealized Gain (Loss) on Debt Securities	(400,000)		(400,000)
Defined Benefit Pension and Other Postretirement Plans	200,000		200,000
Net Income (Loss) Attributable to ALLETE	66,300,000			66,300,000
Common Stock Dividends	31,900,000			(31,900,000)
Net Loss Attributable to Non-Controlling Interest	(1,800,000)				(1,800,000)
Balance, End of Period at Mar. 31, 2020	$ 2,373,000,000.0	$ 1,441,700,000	$ (23,800,000)	$ 853,200,000	$ 101,900,000
Consolidated Statement of Shareholders' Equity [Roll Forward]
Dividends Per Share of Common Stock	$ 0.6175