Common Stock and Earnings Per Share (Tables)

12 Months Ended

Dec. 31, 2017

Equity [Abstract]

Summary of Common Stock [Table Text Block]

Summary of Common Stock	Shares		Equity
	Thousands		Millions
Balance as of December 31, 2014	45,929			$1,107.6
Employee Stock Purchase Plan	18		0.9
Invest Direct	383		19.0
Options and Stock Awards	43		8.6
Equity Issuance Program	1,289		69.9
Forward Sale Agreement and Issuance	1,413		65.4
Balance as of December 31, 2015	49,075		1,271.4
Employee Stock Purchase Plan	16		0.9
Invest Direct	344		20.0
Options and Stock Awards	65		3.7
Contributions to RSOP	60		3.3
Equity Issuance Program	130		8.0
Received for Sale of Land Inventory	(130	)	(8.0		)
Acquisition of Non-Controlling Interest	—		(4.0		)
Balance as of December 31, 2016	49,560		1,295.3
Employee Stock Purchase Plan	12		0.8
Invest Direct	257		19.0
Options and Stock Awards	22		3.6
Contributions to RSOP	50		3.5
Equity Issuance Program	1,000		65.7
Contributions to Pension	216		13.5
Balance as of December 31, 2017	51,117			$1,401.4

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
2017
Net Income Attributable to ALLETE		$172.2			$172.2
Average Common Shares	50.8		0.2	51.0
Earnings Per Share		$3.39			$3.38
2016
Net Income Attributable to ALLETE		$155.3			$155.3
Average Common Shares	49.3		0.2	49.5
Earnings Per Share		$3.15			$3.14
2015
Net Income Attributable to ALLETE		$141.1			$141.1
Average Common Shares	48.3		0.1	48.4
Earnings Per Share		$2.92			$2.92