Quarterly Financial Data (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2018

Selected Quarterly Financial Information [Abstract]

Quarterly Financial Data (Unaudited) [Table Text Block]

Quarter Ended	Mar. 31		Jun. 30		Sept. 30		Dec. 31
Millions Except Earnings Per Share
2018
Operating Revenue		$358.2		$344.1		$348.0		$448.3
Operating Income		$57.4		$36.5		$43.3		$64.0
Net Income Attributable to ALLETE		$51.0		$31.3		$30.7		$61.1
Earnings Per Share of Common Stock
Basic		$1.00		$0.61		$0.59		$1.19
Diluted		$0.99		$0.61		$0.59		$1.18
2017
Operating Revenue		$365.6		$353.3		$362.5		$337.9
Operating Income		$71.6		$54.0		$68.0		$32.3
Net Income Attributable to ALLETE		$49.0		$36.9		$44.9		$41.4
Earnings Per Share of Common Stock
Basic		$0.97		$0.73		$0.88		$0.81
Diluted		$0.97		$0.72		$0.88		$0.81
2016
Operating Revenue		$333.8		$314.8		$349.6		$341.5
Operating Income		$65.2		$40.6		$51.8		$59.4
Net Income Attributable to ALLETE		$45.9		$24.8		$40.3		$44.3
Earnings Per Share of Common Stock
Basic		$0.93		$0.50		$0.82		$0.89
Diluted		$0.93		$0.50		$0.81		$0.89