Common Stock and Earnings Per Share (Tables)

12 Months Ended

Dec. 31, 2020

Equity [Abstract]

Summary of Common Stock [Table Text Block]

Summary of Common Stock	Shares	Equity
	Thousands	Millions
Balance as of December 31, 2017	51,117	$1,401.4
Employee Stock Purchase Plan	11	0.8
Invest Direct	277	20.7
Options and Stock Awards	57	2.1
Contributions to RSOP	47	3.5
Balance as of December 31, 2018	51,509	1,428.5
Employee Stock Purchase Plan	8	0.7
Invest Direct	38	3.0
Options and Stock Awards	85	1.3
Contributions to RSOP	39	3.2
Balance as of December 31, 2019	51,679	1,436.7
Employee Stock Purchase Plan	13	0.7
Invest Direct	309	18.3
Options and Stock Awards	35	2.2
Contributions to RSOP	49	3.0
Balance as of December 31, 2020	52,085	$1,460.9

Reconciliation of Basic and Diluted Earnings Per Share [Table Text Block]

Reconciliation of Basic and Diluted
Earnings Per Share		Dilutive
Year Ended December 31	Basic	Securities	Diluted
Millions Except Per Share Amounts
2020
Net Income Attributable to ALLETE	$174.2		$174.2
Average Common Shares	51.9	—	51.9
Earnings Per Share	$3.36		$3.35
2019
Net Income Attributable to ALLETE	$185.6		$185.6
Average Common Shares	51.6	0.1	51.7
Earnings Per Share	$3.59		$3.59
2018
Net Income Attributable to ALLETE	$174.1		$174.1
Average Common Shares	51.3	0.2	51.5
Earnings Per Share	$3.39		$3.38