Acquisitions - Acquisition of Non-Controlling Interest (Details) - USD ($) $ in Millions		12 Months Ended
	Apr. 15, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Acquisition of Non-Controlling Interest [Line Items]
Payments to Acquire Additional Interest in Subsidiaries		$ 8.0	$ 0.0	$ 6.0
ALLETE Clean Energy [Member]
Acquisition of Non-Controlling Interest [Line Items]
Payments to Acquire Additional Interest in Subsidiaries	$ 8.0
Gain or Loss Recognized in Net Income or Other Comprehensive Income	$ 0.0